Panamanian authorities said it might take a week to search the ship, since so far they have only examined one of its five container sections. They have requested help from United Nations inspectors, along with Colombia and Britain, said Javier Carballo, Panama's top narcotics prosecutor. North Korea is barred by U.N. sanctions from importing sophisticated weapons or missiles.
Panamanian President Ricardo Martinelli said Tuesday that the ship identified as the 14,000-ton Chong Chon Gang, which had departed Cuba en route to North Korea, was carrying missiles and other arms "hidden in containers underneath the cargo of sugar."
Martinelli tweeted a photo showing a green tube that appears to be a horizontal antenna for the SNR-75 "Fan Song" radar, which is used to guide missiles fired by the SA-2 air-defence system found in former Warsaw Pact and Soviet-allied nations, said Neil Ashdown, an analyst for IHS Jane's Intelligence.
"It is possible that this could be being sent to North Korea to update its high-altitude air-defence capabilities," Ashdown said. Jane's also said the equipment could be headed to North Korea to be upgraded.
North Korea has not commented on the seizure, during which 35 North Koreans were arrested after resisting police efforts to intercept the ship in Panamanian waters last week, according to Martinelli. He said the captain had a heart attack and also tried to commit suicide.
But Cuba's Foreign Ministry released a statement late Tuesday acknowledging that the military equipment belonged to the Caribbean nation, saying it had been shipped out to be repaired and returned to the island.
"The agreements subscribed by Cuba in this field are supported by the need to maintain our defensive capacity in order to preserve national sovereignty," the statement read.
It said the vessel was bound for North Korea mostly loaded with sugar — 10,000 tons of it — but added that the cargo also included 240 metric tons of "obsolete defensive weapons": two Volga and Pechora anti-aircraft missile systems, nine missiles "in parts and spares," two Mig-21 Bis and 15 engines for those airplanes.
It concluded by saying that Havana remains "unwavering" in its commitment to international law, peace and nuclear disarmament.
The U.N. Security Council has imposed four rounds of increasingly tougher sanctions against North Korea since its first nuclear test on Oct. 9, 2006.
Under current sanctions, all U.N. member states are prohibited from directly or indirectly supplying, selling or transferring all arms, missiles or missile systems and the equipment and technology to make them to North Korea, with the exception of small arms and light weapons.
The most recent resolution, approved in March after Pyongyang's latest nuclear test, authorizes all countries to inspect cargo in or transiting through their territory that originated in North Korea, or is destined to North Korea if a state has credible information the cargo could violate Security Council resolutions.
"Panama obviously has an important responsibility to ensure that the Panama Canal is utilized for safe and legal commerce," said Acting U.S. Ambassador Rosemary DiCarlo, who is the current Security Council president. "Shipments of arms or related material to or from Korea would violate Security Council resolutions, three of them as a matter of fact."
Panamanian authorities believed the ship was returning from Havana on its way to North Korea, Panamanian Public Security Minister Jose Raul Mulino told The Associated Press. Based on unspecified intelligence, authorities suspected it could be carrying contraband and tried to communicate with the crew, who didn't respond. Martinelli said Panama originally suspected drugs could be aboard.
"Panama being a neutral country, a country in peace, that doesn't like war, we feel very worried about this military material," Martinelli said.
In early July, a top North Korean general, Kim Kyok Sik, visited Cuba and met with his island counterparts. The Cuban Communist Party newspaper Granma said he was also received by President Raul Castro, and the two had an "exchange about the historical ties that unite the two nations and the common will to continue strengthening them."
The meetings were held behind closed doors, and there has been no detailed account of their discussions.
"After this incident there should be renewed focus on North Korean-Cuban links," said Hugh Griffiths, an arms trafficking expert at the Stockholm International Peace Research Institute. Griffiths said his institute told the U.N. this year that it had uncovered evidence of a flight from Cuba to North Korea that travelled via central Africa.
"Given the history of North Korea, Cuban military co-operation and now this latest seizure, we find this flight more interesting," he said. "
The Chong Chon Gang has a history of being detained on suspicion of trafficking drugs and ammunition, Griffiths said. Lloyd's List Intelligence said the 34-year-old ship, which is registered to the Pyongyang-based Chongchongang Shipping Company, "has a long history of detentions for safety deficiencies and other undeclared reasons."
Griffiths said the Chong Chon Gang was stopped in 2010 in the Ukraine and was attacked by pirates 400 miles (640 kilometres) off the coast of Somalia in 2009.
Griffiths' institute has also been interested in the ship because of a 2009 stop it made in Tartus — a Syrian port city hosting a Russian naval base.
Follow Michael Weissenstein on Twitter at http://twitter.com/mweissenstein
AP writers Michael Weissenstein from Mexico City, Arnulfo Franco in Manzanillo, Panama, Malin Rising in Stockholm, Peter Orsi in Havana, and Edith M. Lederer and Ron DePasquale at the United Nations contributed to this report.