South Korea on Wednesday welcomed the seizure off Panama of a North Korean ship suspected of carrying weapons and urged the United Nations to take up the case.

"The government appreciates the Panamanian government's stoppage of a North Korea vessel carrying a suspicious cargo," the foreign ministry said in a statement.

"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 ministry, however, declined to confirm a report by Yonhap news agency that Panama informed Seoul that military equipment on the ship appeared to be missile parts banned under UN resolutions.

Panama said Tuesday it had found military equipment believed to be missiles concealed in a cargo of sugar, after impounding the ship and conducting a search for drugs.

It 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 and missile programmes.

Cuba said the weapons found on the ship were "obsolete" Soviet-era arms that it had sent to North Korea for repair.

It said the 240 tonnes of "obsolete defensive weapons" included two anti-aircraft missile systems.

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".

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", Shin In-Kyun, president of the private Korea Defence Network, told AFP.

Lee Ho-Ryung, an analyst with the Korea Institute for Defence Analyses, said she was sceptical about Cuba's announcement.

"It's hard to understand the North taking such high risks just to repair another country's missiles at a time when it is under tight international sanctions," Lee said.

UN sanctions bar the transport of all weapons to and from North Korea apart from imports 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.

Defence and security consultants IHS Jane's said the ship stopped in Panama was carrying a radar that guides surface-to-air missiles to their target.

"IHS Jane's has identified the equipment shown in the images so far released as an RSN-75 'Fan Song' fire control radar for the SA-2 family of surface-to-air missiles," the group said in a statement.

SA-2 missile systems date from the Soviet era and include a long-range radar that detects targets on approach and a fire control radar that guides missiles towards their target, said Jeremy Binnie, an IHS expert.

"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."

But it says a second scenario could be that the fire control radar equipment may have been en route to North Korea to augment Pyongyang's air defence network.

The North has yet to comment on the Panama incident.

The impoverished country has for decades been developing missiles with varying capabilities.

It said its launch last December was designed only to put a peaceful satellite into orbit.

But the United Nations saw it as a covert test of ballistic missile technology, which is banned under UN resolutions.