Tokyo Electric Power Company workers have detected high levels of radiation in a ditch that flows into the ocean from a leaking tank at the crippled Fukushima Daiichi nuclear plant.

Japan's nuclear watchdog said Thursday the leak could be the beginning of a new disaster — a series of leaks of contaminated water from hundreds of steel tanks holdng massive amounts of radioactive water coming from three melted reactors, as well as underground water running into reactor and turbine basements.

Tokyo Electric Power Co. says about 300,000 litres of contaminated water leaked from one of the tanks, possibly through a seam. The leak is the fifth, and worst, since last year involving tanks of the same design at the wrecked Fukushima Dai-ichi plant, raising concerns that contaminated water could begin leaking from storage tanks one after another.

"That's what we fear the most. We must remain alert. We should assume that what has happened once could happen again, and prepare for more," Nuclear Regulation Authority chairman Shunichi Tanaka told a news conference. "We are in a situation where there is no time to waste."

The watchdog also proposed at a weekly meeting Wednesday to raise the rating of the seriousness of the leak to level three, a "serious incident," from level one, "an anomaly," on an International Nuclear and Radiological event scale from zero to seven.

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  • This Tuesday, Aug. 20, 2013 aerial photo shows the Fukushima Dai-ichi nuclear plant at Okuma in Fukushima prefecture, northern Japan. The Japanese nuclear watchdog proposed Wednesday to define a fresh leakage of highly radioactive leak from one of the hundreds of storage tanks at Japan’s crippled atomic power plant this week, its worst leak yet from such a vessel. The operator of the Fukushima Dai-ichi plant said Tuesday said about 300 tons (300,000 liters, 80,000 gallons) of contaminated water have leaked from a steel storage tank at the wrecked Fukushima Dai-ichi plant. TEPCO hasn’t figured out how or where the water leaked, but suspects it did so through a seam on the tank or a valve connected to a gutter around the tank. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT

  • In this Tuesday, Aug. 20, 2013 aerial photo, workers stand on storage tanks at the Fukushima Dai-ichi nuclear plant at Okuma in Fukushima prefecture, northern Japan. Deep beneath Fukushima’s crippled nuclear power station a vast underground reservoir of highly contaminated water that began spilling from the plant’s reactors during the 2011 earthquake and tsunami has been creeping slowly toward the sea. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT

  • In this Tuesday, Aug. 20, 2013 aerial photo, workers stand on storage tanks at the Fukushima Dai-ichi nuclear plant at Okuma in Fukushima prefecture, northern Japan. The Japanese nuclear watchdog proposed Wednesday to define a fresh leakage of highly radioactive leak from one of the hundreds of storage tanks at Japan’s crippled atomic power plant this week, its worst leak yet from such a vessel. The operator of the Fukushima Dai-ichi plant said Tuesday said about 300 tons (300,000 liters, 80,000 gallons) of contaminated water have leaked from a steel storage tank at the wrecked Fukushima Dai-ichi plant. TEPCO hasn’t figured out how or where the water leaked, but suspects it did so through a seam on the tank or a valve connected to a gutter around the tank. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT

  • This Aug. 20, 2013 aerial photo shows the Fukushima Dai-ichi nuclear plant at Okuma in Fukushima prefecture, northern Japan. The Japanese nuclear watchdog proposed Wednesday to define a fresh leakage of highly radioactive leak from one of the hundreds of storage tanks at Japan’s crippled atomic power plant this week, its worst leak yet from such a vessel. The operator of the Fukushima Dai-ichi plant said Tuesday said about 300 tons (300,000 liters, 80,000 gallons) of contaminated water have leaked from a steel storage tank at the wrecked Fukushima Dai-ichi plant. TEPCO hasn’t figured out how or where the water leaked, but suspects it did so through a seam on the tank or a valve connected to a gutter around the tank. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT

  • This Tuesday, Aug. 20, 2013 aerial photo shows the Fukushima Dai-ichi nuclear plant at Okuma in Fukushima prefecture, northern Japan. Deep beneath Fukushima’s crippled nuclear power station a vast underground reservoir of highly contaminated water that began spilling from the plant’s reactors during the 2011 earthquake and tsunami has been creeping slowly toward the sea. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT

  • OKUMA, JAPAN - AUGUST 19: (EDITORIAL USE ONLY) In this handout image provided by Tokyo Electric Power Co, leaked radioactive water remains near the tank at the Fukushima Daiichi Nuclear Power Plant on August 19, 2013 in Okuma, Fukushima, Japan. An estimated 300 tons of highly radioactive water has leaked from a tank at the Fukushima nuclear plant, with much of the polluted water apparently seeping into the ground. (Photo by Tokyo Electric Power Co via Getty Images)

  • In this Wednesday, June 12, 2013 file photo, a construction worker walks beside the underground water tanks at the Fukushima Dai-ichi nuclear plant at Okuma in Fukushima prefecture, Japan. The operator of Japan's tsunami-crippled nuclear power plant said Tuesday, Aug. 20, 2013, that about 300 tons of highly radioactive water have leaked from one of the hundreds of storage tanks there - its worst leak yet from one of the vessels. (AP Photo/Toshifumi Kitamura, Pool)

  • OKUMA, JAPAN - AUGUST 19: (EDITORIAL USE ONLY) In this handout image provided by Tokyo Electric Power Co, leaked radioactive water remains near the tank at the Fukushima Daiichi Nuclear Power Plant on August 19, 2013 in Okuma, Fukushima, Japan. An estimated 300 tons of highly radioactive water has leaked from a tank at the Fukushima nuclear plant, with much of the polluted water apparently seeping into the ground. (Photo by Tokyo Electric Power Co via Getty Images)

  • FILE - In this Feb. 10, 2013 file photo, tens of cylindrical tanks built for storage of polluted water are seen near the four reactor buildings, background, at the tsunami-devastated Fukushima Dai-Ichi nuclear plant in Okuma, northern Japan, where preparations for dismantlement of the facilities are underway. Nuclear Regulation Authority Chairman Shunichi Tanaka said Wednesday, April 10, 2013 that leaks of radioactive water from underground tanks are undermining efforts to decommission the plant. The plant's operator Tokyo Electric Power Co. said three of the seven underground tanks are leaking, but that the contaminated water is not believed to have reached the ocean. However, experts suspect that water has leaked steadily into the sea since early in the crisis, citing high radiation levels in fish in waters off the plant. (AP Photo/Kyodo News, File) JAPAN OUT, MANDATORY CREDIT, NO LICENSING IN CHINA, HONG KONG, JAPAN, SOUTH KOREA AND FRANCE

  • FILE - This March 11, 2012 file photo shows storage tanks for radiation-contaminated water in the compound of the tsunami-crippled Fukushima Dai-ichi nuclear power plant in Okuma, Fukushima Prefecture, northeastern Japan. Tokyo Electric power Co. (TEPCO), the operator of the plant said Saturday, April 6, 2013 that it was moving tons of highly radioactive water from a temporary storage tank to another after detecting signs of leakage, in a blow to the plant's struggles with tight storage space. TEPCO said about 120 tons of the water are believed to have breached the tank's inner linings, some of it possibly leaking into the soil. TEPCO is moving the water to a nearby tank at the plant - a process that could take several days. (AP Photo/Kyodo News, File) JAPAN OUT, MANDATORY CREDIT, NO LICENSING IN CHINA, HONG KONG, JAPAN, SOUTH KOREA AND FRANCE

  • FILE - In this Wednesday, March 6, 2013 file photo, workers wearing protective gears take a survey near tanks of radiation contaminated water at Tokyo Electric Power Co.'s Fukushima Dai-ichi nuclear power plant in Okuma town, Fukushima prefecture, northeast of Tokyo. The operator of the wrecked Fukushima nuclear power plant says it is struggling with its latest efforts to stop contaminated underground water leaks from running into the sea. TEPCO said Tuesday, Aug. 6 that some of the water was seeping over or around "chemical walls" it had created by injecting chemicals into the soil that solidify into a wall. (AP Photo/Issei Kato, Pool, File)

  • This photo taken on August 6, 2013 shows local government officials and nuclear experts inspecting a construction site to prevent the seepage of contamination water into the sea, at Tokyo Electric Power's (TEPCO) Fukushima Dai-ichi nuclear plant in Okuma, Fukushima prefecture. Japan will accelerate efforts to prevent more radioactive groundwater from seeping into the ocean at the crippled Fukushima nuclear plant, government officials said on August 7, as critics slam its operator's handling of the issue. JAPAN OUT AFP PHOTO / JAPAN POOL via JIJI PRESS (Photo credit should read JAPAN POOL/AFP/Getty Images)

  • This aerial photo taken on July 9, 2013 shows reactor buildings Unit 2, left, and Unit 1 at Fukushima Dai-ichi nuclear power plant in Okuama, Fukushima Prefecture, northern Japan. Japan's nuclear regulator says radioactive water from the crippled Fukushima power plant is probably leaking into the Pacific Ocean, a problem long suspected by experts but denied by the plant's operator. Officials from the Nuclear Regulation Authority said a leak is "strongly suspected" and urged plant operator Tokyo Electric Power Co. to determine where the water may be leaking from and assess the environmental and other risks, including the impact on the food chain. The watchdog said Wednesday, July 10, 2013 it would form a panel of experts to look into ways to contain the problem. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT

  • FILE - In this Wednesday, June 12, 2013 file photo, the steel structure for the use of the spent fuel removal from the cooling pool is seen at the Unit 4 of the Fukushima Dai-ichi nuclear plant at Okuma in Fukushima prefecture, Japan. Tokyo Electric Power Co., the operator of Japan’s crippled nuclear power plant, showed journalists the massive steel structure built next to one of the damaged reactor buildings to help extract more than 1,500 fuel rods from a cooling pool on top of it. Japan's nuclear watchdog has formally approved new safety requirements for atomic plants, paving the way for the reopening of facilities shut down since the Fukushima disaster. The new requirements approved Wednesday, June 19, by the Nuclear Regulation Authority will take effect on July 8, when operators will be able to apply for inspections. If plants pass inspection, they can reopen. (AP Photo/Noboru Hashimto, Pool, File)

  • In this Wednesday, June 12, 2013 photo, the crippled Fukushima Dai-ichi nuclear power plant is seen through a bus window in Okuma, in Fukushima prefecture. More than two years after Japan’s nuclear disaster, damaged vehicles, twisted metal and other debris remain strewn about the Fukushima plant. Japan's nuclear watchdog has formally approved new safety requirements for atomic plants, paving the way for the reopening of facilities shut down since the Fukushima disaster. The new requirements approved Wednesday, June 19, by the Nuclear Regulation Authority will take effect on July 8, when operators will be able to apply for inspections. If plants pass inspection, they can reopen. (AP Photo/Noboru Hashimoto, Pool)

The watchdog urged TEPCO to step up monitoring for leaks and take precautionary measures.

During the meeting, officials also revealed that plant workers apparently have overlooked several signs of leaks, suggesting their twice-daily patrols were largely just a walk. They have not monitored water levels inside tanks, obviously missed a puddle forming at the bottom of the tank earlier, and kept open a valve on an anti-leakage barrier around the tanks.

TEPCO said the leaked water is believed to have mostly seeped into the ground after escaping from the barrier around the tank. It initially said the leak did not pose an immediate threat to the sea because of its distance — about 500 metres from the coastline. But TEPCO reversed that view late Wednesday and acknowledged a possible leak to the sea after detecting high radioactivity inside a gutter extending to the ocean.

The company also said the tank may have been leaking slowly for weeks through a possible flaw in its bottom. That could create extensive soil contamination and a blow to plans to release untainted underground water into the sea as part of efforts to reduce the amount of radioactive water.

The leaks have shaken confidence in the reliability of hundreds of tanks that are crucial for storing water that has been pumped into the broken reactors to keep melted radioactive fuel cool.

Multiple meltdowns

The plant suffered multiple meltdowns following a massive earthquake and tsunami in March 2011 — a level seven "major accident" and the worst since Chernobyl, in Ukraine, in 1986.

About 350 of about 1,000 steel tanks built across the plant complex containing nearly 300 million litres of partially treated contaminated water are less-durable ones with rubber seams.

TEPCO says the tanks that have leaked use rubber seams that were intended to last about five years. TEPCO spokesman Masayuki Ono said it plans to build additional tanks with welded seams that are more watertight, but will still have to rely on ones with rubber seams.

Figuring out what to do with the radioactive water is among the most pressing issues affecting the cleanup process, which is expected to take decades.

The leaked water's radiation level, measured about 50 centimetres above the puddle, was about 100 millisieverts per hour — the maximum cumulative exposure allowed for plant workers over five years, Ono said.

Contaminated water that TEPCO has been unable to contain continues to enter the Pacific Ocean at a rate of hundreds of tons per day. Much of that is ground water that has mixed with untreated radioactive water at the plant.

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  • A woman prays during a rally against nuclear power plants as victims of the March 11, 2011 earthquake and tsunami are remembered, at a park in Tokyo, Monday, March 11, 2013. Japan marked the second anniversary on Monday of the devastating earthquake and tsunami that left nearly 19,000 people dead or missing. (AP Photo/Itsuo Inouye)

  • People observe a moment of silence for the victims of the March 11, 2011 earthquake and tsunami during an event at a park in Tokyo, Monday, March 11, 2013. Japan marked the second anniversary on Monday of the devastating earthquake and tsunami that left nearly 19,000 people dead or missing. (AP Photo/Itsuo Inouye)

  • Gathering around what is left of a disaster control center devastated by the March 11, 2011 earthquake and tsunami, people bow their heads Monday, March 11, 2013 in Minamisanriku, Miyagi prefecture, Japan, in a moment of silence at 2:46 p.m. when the magnitude 9.0 earthquake struck off Japan's northeastern coast. Japan marked the second anniversary on Monday of a devastating disasters that left nearly 19,000 people dead or missing. (AP Photo/Shizuo Kambayashi)

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    People offer prayer in a moment of silence in front of what is left of a disaster control center in an area devastated by the March 11, 2011 earthquake and tsunami in Minamisanriku, Miyagi Prefecture, Monday, March 11, 2013. Japan marked the second anniversary on Monday of the devastating disasters that left nearly 19,000 people dead or missing. (AP Photo/Shizuo Kambayashi)

  • People observe a moment of silence in front of what is left of a disaster control center in an area devastated by the March 11, 2011 earthquake and tsunami, in Minamisanriku, Miyagi prefecture, in Japan, Monday, March 11, 2013. Japan marked the second anniversary on Monday of the devastating earthquake and tsunami that left nearly 19,000 people dead or missing.(AP Photo/Shizuo Kambayashi)

  • People offer prayers in front of what is left of a disaster control center in an area devastated by the March 11, 2011 earthquake and tsunami, in Minamisanriku, Miyagi Prefecture, Monday, March 11, 2013. Japan marked the second anniversary on Monday of the devastating disasters that left nearly 19,000 people dead or missing. (AP Photo/Shizuo Kambayashi)

  • A worker walks behind a white plastic plate cutout of the words of a local businessperson, declaring determination to reopen the business, in a tsunami-stricken area in Minamisanriku, Miyagi prefecture, northern Japan, Monday, March 11, 2013. Japan marked the second anniversary on Monday of a devastating earthquake and tsunami that left nearly 19,000 people dead or missing. The words mean; I will restart my business from zero like my ancestor. (AP Photo/Shizuo Kambayashi)

  • A man points to the name of one of the victims of the earthquake and tsunami inscribed in a cenotaph in Okawa district in Ishinomaki, Miyagi Prefecture, northern Japan, Monday, March 11, 2013. Japan marked the second anniversary on Monday of a devastating earthquake and tsunami that left nearly 19,000 people dead or missing. (AP Photo/Shizuo Kambayashi)

  • People observe a moment of silence for the victims of the March 11, 2011 earthquake and tsunami during an event at a park in Tokyo, Monday, March 11, 2013. (AP Photo/Itsuo Inouye)

  • Aerial

    Red and white cranes stand by reactors of the tsunami-ravaged Fukushima Dai-ichi nuclear power plant in Fukushima Prefecture, northeastern Japan, Monday morning, March 11, 2013. The two-year anniversary Monday of Japan’s devastating earthquake, tsunami and nuclear catastrophe is serving to spotlight the stakes of the country’s struggles to clean up radiation, rebuild lost communities and determine new energy and economic strategies. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT, NO LICENSING IN CHINA, HONG KONG, JAPAN, SOUTH KOREA AND FRANCE

  • People offer prayers as a Buddhist monk chants a sutra for tsunami victims to mark the second anniversary of the 2011 earthquake an tsunami on a beach in Arahama in Sendai, Miyagi Prefecture, Japan, Monday morning, March 11, 2013. Japan marked the second anniversary of the disasters, that killed nearly 19, 000 people in areas along Japan's northeastern coast. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT, NO LICENSING IN CHINA, HONG KONG, JAPAN, SOUTH KOREA AND FRANCE

  • An artificially-restored "miracle pine tree," that survived the March 11, 2011 tsunami, is silhouetted against the rising sun in Rikuzentakata, Iwate prefecture, Japan, Monday, March 11, 2013. Japan marked the second anniversary of its earthquake, tsunami and nuclear catastrophe, that killed nearly 19, 000 people in areas along Japan's northeastern coast. The 27-meter (88-foot and 7-inch)-tall tree, a single survivor among 70,000 trees in a forest along the coast, has just been restored in a project to preserve it. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT, NO LICENSING IN CHINA, HONG KONG, JAPAN, SOUTH KOREA AND FRANCE

  • People offer prayers in front of a memorial cenotaph for tsunami victims on a beach in Arahama in Sendai, Miyagi Prefecture, Japan, Monday morning, March 11, 2013. Japan marked the second anniversary of its earthquake, tsunami and nuclear catastrophe, that killed nearly 19, 000 people in areas along Japan's northeastern coast. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT, NO LICENSING IN CHINA, HONG KONG, JAPAN, SOUTH KOREA AND FRANCE

  • Police officers search for the remains of those who went missing in the March 11, 2011 tsunami on the coastline in Ishinomaki, Miyagi Prefecture, northern Japan, Monday, March 11, 2013. The two-year anniversary Monday of Japan's devastating earthquake, tsunami and nuclear catastrophe is serving to spotlight the stakes of the country's struggles to clean up radiation, rebuild lost communities and determine new energy and economic strategies.(AP Photo/Shizuo Kambayashi)

  • Women approach Okawa Elementary School where 74 of the 108 students went missing after the March 11 tsunami in Ishinomaki, Miyagi prefecture, Japan, Monday, March 11, 2013. The two-year anniversary Monday of Japan's devastating earthquake, tsunami and nuclear catastrophe is serving to spotlight the stakes of the country's struggles to clean up radiation, rebuild lost communities and determine new energy and economic strategies. (AP Photo/Shizuo Kambayashi)