The seizure off Panama of a North Korean ship suspected of carrying weapons is a symptom of a vital barter trade that Pyongyang has managed to keep largely hidden, a Stockholm-based expert said Wednesday.

It is significant that the seized items, believed to be missile parts, were concealed in a cargo of Cuban sugar, according to Hugh Griffiths, a specialist on illicit trade at the Stockholm Peace Research Institute.

It is likely to be an example of a barter trade of unknown magnitude in which North Korea offers repair of military equipment in return for basic food stuffs despite UN sanctions against Pyongyang, said Griffiths.

"Most of it slips under the radar. Attention focuses on North Korea's ballistic missile capabilities and its nuclear capabilities, but most of its foreign trade is actually in conventional arms with a small group of countries," he said.

In the past these trading partners have included such countries as Myanmar, Eritrea and Yemen, which are not quite as isolated as North Korea, but are poverty-stricken and tend to be run as some form of dictatorship, he said.

"Within this context they need to trade, and North Korea has the technicians that can handle machinery both on the civilian and military side, so it's a natural match in many ways," he said.

He described North Korea as "a very highly militarised society, for whom the main export is conventional military equipment with very little else to offer".

North Korea has become adept at disguising this trade, often transporting the items in containers carried by respectable shipping companies that have no idea what is actually inside, he said.

"It's very anonymous and hard to identify. Globalisation and containerisation have made trade easier but also made trafficking easier," he said.

Giving an exact figure for the extent of the trade is impossible, as North Korea is one of the world's least transparent countries, and barter trade leaves no financial tracks anyway, according to Griffiths.

"The North Koreans used to be secretive, but now they are secretive for an even better reason than before," he said.

The best way to stop the practice is to improve information sharing and cooperation among UN member states in order to implement sanctions in a more meaningful manner, he said, adding that it might be tough to get all on the same page.

"We previously did a study commissioned by the UN Sanctions Committee on air transport to and from North Korea over the past eight years, so we sent out a great number of questionnaires to states to get their traffic data. We didn't receive any replies from quite a number of key states," he said.