North Korea has broken its silence over a ship's cargo of arms seized in a dramatic raid by Panama, demanding the vessel's release, as its mutinous crew members refuse to cooperate with investigators.

The communist state rebuffed suspicions of UN sanctions-busting, saying the shipment on the North Korean-flagged vessel formed part of a legitimate deal with Cuba.

Havana said the military equipment, discovered on the Chong Chon Gang among tonnes of sugar, was "obsolete" Soviet-era missiles and parts that were being sent to North Korea for repair -- an account backed up by Pyongyang.

"This cargo is nothing but ageing weapons which are to be sent back to Cuba after overhauling them according to a legitimate contract," said a foreign ministry statement carried by the North's Korean Central News Agency.

"The Panamanian authorities should take a step to let the apprehended crewmen and ship leave without delay," added the statement, North Korea's first on the affair.

Panama on Monday said it had discovered the shipment after impounding the vessel and conducting a drugs search. The ship's captain tried to kill himself and the crew rioted, according to Panama's president.

The 35-man crew on Wednesday refused to answer questions about the cargo.

"They took up their right not to testify," Panamanian anti-drugs prosecutor Javier Caraballo told reporters in Manzanillo, where police are still unloading sacks of sugar, looking for any more military equipment hidden on the boat.

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

On Wednesday Panama officially requested United Nations inspectors scrutinise the cargo.

"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 Anything that is not logged, even if it is obsolete, is contraband," Panamanian Security Minister Jose Raul Mulino said.

UN diplomats said strict arms sanctions imposed on Pyongyang over its nuclear programme could have been breached.

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

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

Cuba says the cargo comprised missile parts and spares, Mig-21 aircraft parts and 15 plane motors, all manufactured "in the mid-20th century" and "to be repaired and returned to Cuba".

South Korea has welcomed the seizure, while media in Seoul reported that three years ago the Chong Chon Gang was detained in Ukraine for carrying narcotics and other contraband.

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

Analysts said the seizure shone a spotlight on a secretive barter trade where Pyongyang repairs military equipment in exchange for food.

Defence and security consultants IHS Jane's said it was possible the shipment had been on its way 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."

Chang Yong-Seok of the Institute for Peace and Unification at Seoul's National University told AFP that the discovery was "just the tip of the iceberg".

"In the case of Cuba, North Korea has been involved in this type of trade since the 1960s when Pyongyang reached out to non-aligned countries," he said.

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.