Finland has impounded a ship bound for China carrying 69 surface-to-air Patriot missiles.

The missiles, produced by US firm Raytheon, were discovered following a customs search on the British-registered Thor Liberty, owned by Danish firm Thorco, at the port of Kotka, about 120 kilometres from Helsinki.

The BBC is reporting the missiles were found in containers marked fireworks.

A team of customs, police and defence force experts conducted a detailed probe of the ship's cargo on Wednesday when routine checks by Finland's traffic safety authority revealed a load of 150 tons of improperly packed nitroguanidine - a low-sensitivity explosive with a high detonation speed.

Following the discovery, police and customs are now conducting further investigations to determine the reason for the military cache.

Finnish customs are investigating the case as one of illegal export of defence material.

"There are grounds for customs to open an investigation into crime concerning the export of these items to third countries," head of Finnish customs anti-crime unit Petri Lounatmaa said.

Finnish law requires permission from defence officials to move such material across the country's borders.

Mr Lounatmaa noted that Finnish police, who are part of the investigating team, will investigate any other possible misconduct in the case.

"We have started questioning the crew...as the investigation continues decisions will be made about possible arrests," he said.

The customs official said that there were roughly 32 crew members on board the vessel, and that questioning could continue into Friday.

Thorco managing director Thomas Mikkelsen expressed surprise, telling reporters from Denmark that he was unaware of the case.

Another company official, speaking on condition of anonymity, confirmed the ship had been detained in Finland and said the missiles could have been loaded on to the vessel by mistake.

AFP