A North Korean cargo ship stopped in Panama was carrying a radar that guides surface-to-air missiles to target, defence and security consultants IHS Jane's said Tuesday.

Panama announced Monday that it had intercepted the vessel Chong Chon Gang, which was en route from Cuba -- one of Pyongyang's only allies -- on suspicion it could be smuggling drugs.

Instead, authorities found suspected missile material on board.

"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 Union, 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 existing air defence network.

UN sanctions bar the transport of all weapons to or from North Korea apart from small arms. Several of the country's ships have been searched in recent years.

The North has for decades developed missiles of all types, and it also has a nuclear programme that has caused global concern.

In February, Pyongyang defiantly carried out its third nuclear weapons test and then threatened to attack the United States, in language that was shrill even by its standards.

Last December it also successfully launched a three-stage rocket which placed a satellite in orbit.

Pyongyang said the operation was a peaceful scientific mission, but the launch was widely condemned as a covert ballistic missile test banned under UN resolutions.

It is unclear whether the North has the technology to build a nuclear warhead for a missile.