North Korea said Wednesday that Cuban arms seized from a Pyongyang-flagged ship near the Panama Canal were part of a legitimate deal, amid concerns UN sanctions may have been violated.

Havana said the arms, discovered on the ship among tons of sugar, were "obsolete" Soviet-era missiles and parts, which were being sent to North Korea for repair -- an account backed up by its allies in Pyongyang.

"This cargo is nothing but aging weapons which are to be sent back to Cuba after overhauling them according to a legitimate contract," North's Korea Central News Agency quoted the foreign ministry as saying.

"The Panamanian authorities should take a step to let the apprehended crewmen and ship leave without delay," the statement added.

But Panama on Wednesday officially requested UN inspectors scrutinise the cargo, which UN diplomats said could constitute a breach of the strict arms sanctions imposed on North Korea over its nuclear programme.

"The cargo is illegal because it was not declared. Anything that is not logged, even if it is obsolete, is contraband," Panamanian Security Minister Jose Raul Mulino said.

"We are awaiting the arrival of experts from the United States and Britain, per our request, as well as a technical team from the UN Security Council," he added.

A spokesman for UN chief Ban Ki-moon said in a statement the secretary general "commends the action taken by Panama in full conformity with its obligations under the relevant Security Council resolutions."

"The Secretary-General awaits the outcome of the investigation into the matter in question and is sure the 1718 Security Council Sanctions Committee will promptly address it."

Panama said Monday it had discovered the shipment after impounding the Chong Chon Gang and conducting a drugs search.

On Tuesday, Cuba claimed the shipment as its own, with the foreign ministry listing 240 metric tons of "obsolete defensive weapons," including two anti-aircraft missile systems.

There were also "nine missiles in parts and spares," various Mig-21 aircraft parts and 15 plane motors, all manufactured "in the mid-20th century" and "to be repaired and returned to Cuba."

Panama's President Ricardo Martinelli tweeted a photo of the haul, which experts said Tuesday was an aging Soviet-built radar control system for surface-to-air missiles.

South Korea welcomed the seizure.

"If the shipment turns out to be in breach of UN resolutions, we expect the UN Security Council's sanctions committee to take relevant steps expeditiously," Seoul's foreign ministry said.

In Washington, State Department spokeswoman Marie Harf said the US would raise the issue with Havana "very soon".

At the United Nations, diplomats said UN sanctions had likely been violated.

"Clearly the facts still need to be established," said Britain's UN ambassador Mark Lyall Grant, though he added: "On the face of it, the transfer of these weapons to North Korea would be a violation of the sanctions."

Diplomats said Cuba should have sought a waiver to transfer the weapons, if they are obsolete.

They also said North Korea sanctions panel experts would take up the case but could require months to investigate it.

Defense and security consultants IHS Jane's said the shipment could have been sent to North Korea for an upgrade, as Cuba insists.

"In this case, it would likely be returned to Cuba and the cargo of sugar could be a payment for the services."

Panamanian officials said Monday the crew resisted the search and the ship's captain attempted to commit suicide after the vessel was stopped.

North Korea countered that the captain and crew had been "rashly attacked," condemning what it called a "violent action" by Panamanian investigators.

The ship was sailing from Cuba with a crew of about three dozen when it was stopped by drug enforcement officials and taken into port in Manzanillo.

Analysts in Seoul said the North was fully capable of providing missile repair services for other countries.

"But we cannot rule out the possibility of North Korea importing parts for its own Soviet-era missiles," Shin In-Kyun, president of the private Korea Defence Network, told AFP.

North Korea's army chief of staff, General Kyok Sik Kim, visited Cuba last month and said the two countries were "in the same trench".

Pyongyang carried out a third nuclear weapons test in February, triggering tighter UN sanctions, which bar the transport of all weapons to and from North Korea apart from the import of small arms.

Five percent of the world's commerce travels through the century-old Panama Canal, with that expected to increase following the completion of a major expansion project.

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