PANAMA CITY (AFP) - Panama on Wednesday awaited the arrival of Unjted States and British experts to inspect Cuban weapons found aboard a North Korean ship, as Seoul urged the United Nations to investigate.

Havana said the arms discovered on the vessel near the Panama Canal among tons of sugar were "obsolete" Soviet-era missiles and parts it had sent back to Pyongyang for repair.

But Panama has urged UN inspectors to scrutinise the cargo, which could constitute a violation 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 Anything that is not logged, even if it is obsolete, is contraband," Security Minister Jose Raul Mulino said.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.

Panama said on Monday it had discovered military equipment, which it believed to be missiles, after impounding the vessel and conducting a drugs search.

On Tuesday, Cuba, one of North Korea's few allies, claimed the shipment as its own, with the Foreign Ministry listing 240 tonnes of "obsolete defensive weapons", including two anti-aircraft missile systems, as being on board.

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

"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 ministry said in an English-language statement.

Panama President Ricardo Martinelli tweeted a photo of the haul, which experts earlier on Tuesday identified as an ageing Soviet-built radar control system for surface-to-air missiles.

Mr Martinelli's government said the munitions were hidden in a shipment of 100,000kg of bagged sugar aboard the North Korean-flagged Chong Chon Gang.

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," the Foreign Ministry said in Seoul.

The United States also hailed the discovery, and said it would offer assistance if asked.

Defence and security consultants IHS Jane's said Tuesday that the photo tweeted by Martinelli appeared to show an "RSN-75 'Fan Song' fire-control radar system".

"One possibility is that Cuba could be sending the system to North Korea for an upgrade," the group said. "In this case, it would likely be returned to Cuba and the cargo of sugar could be a payment for the services."

North Korea has yet to comment on the incident.

Panamanian officials said on Monday that the crew resisted the search, and that the ship's captain attempted to commit suicide after the vessel was stopped. The vessel and crew were still being held.

Reporters on Tuesday were taken aboard the vessel - a rusting, dingy ship with a dank kitchen. The only reasonably appointed room was the captain's quarters.

It was sailing from Cuba towards the canal with a crew of about three dozen and was stopped by drug enforcement officials and taken into port in Manzanillo.

A Panama government spokesman said an examination of the ship by weapons specialists may take as long as a week.

"The world needs to sit up and take note: you cannot go around shipping undeclared weapons of war through the Panama Canal," Mr Martinelli told Radio Panama on Monday.

Analysts in Seoul said the North, which successfully launched a long-range rocket to put a satellite into orbit last December, 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", Mr 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.

Several of the country's ships have been searched in recent years.

In July 2009, a North Korean ship heading to Myanmar, the Kang Nam 1, was followed by the US Navy due to suspicions it was carrying weapons. It turned around and headed back home.

Five per cent 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.