If confirmed, it would be the 12th such case of piracy since April in Southeast Asia, where tankers have been hijacked, cargo stolen and later released, according to the International Maritime Bureau.
In Vietnam, Nguyen Vu Diep of the Hai Phong Sea Products Shipbuilding company said they lost contact with the Sunrise 689 and its 18 crew members several hours after it left Singapore en route to Vietnam on Oct. 2.
The tanker, which was carrying 5,225 tons of diesel, was last detected about 115 nautical miles (213 kilometres) off Singapore and was due to arrive at Cua Viet port in Quang Tri province early Wednesday, he said.
"There are almost no possibilities of technical failure or bad weather," he told The Associated Press. "It's likely that the tanker was hijacked."
Col. Ngo Ngoc Thu, deputy commander of Vietnam's coast guard, said maritime authorities from Malaysia and Singapore were scouring the area to try to find the tanker.
Noel Choong, who heads the IMB's piracy reporting centre in Kuala Lumpur, said the ship was classified as missing and believed to have been hijacked. He said all its communications equipment appeared to have been switched off or destroyed, and the vessel can't be detected by satellite.
In previous cases, tankers carrying gas oil, diesel oil or lupe oil have been boarded by pirates and taken to an unknown location, and only released later after their cargo was stolen, he said.
The IMB has informed Interpol about this latest case and also urged ships to be on the lookout for the missing vessel and for pirates, he said.
"We just hope that maritime authorities will co-ordinate to find the tanker," Diep said. "The crew members' life could be at risk if they failed to act quickly."
Associated Press writer Minh V. Tran in Hanoi, Vietnam, contributed to this report.