The Finnish authorities have impounded an Isle of Man-flagged ship bound for China with undeclared missiles and explosives, officials say.
Police are questioning the crew of the MS Thor Liberty after what were described as 69 Patriot anti-missile missiles were found aboard.
Interior Minister Paivi Rasanen said the missiles were marked "fireworks".
The MS Thor Liberty had docked in the Finnish port of Kotka after leaving Germany last week.
Dock workers became suspicious after finding explosives poorly stored on open pallets, and the missiles were then found in containers marked "fireworks".
The managing director of the ship's owner, Thorco Shipping, expressed surprise. Thomas Mikkelsen told AFP news agency from Denmark that he was unaware of the matter.
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 adds.
Police did not confirm Finnish media reports that the ship had also been scheduled to stop in South Korea, Reuters news agency reports.
The MS Thor Liberty left port in Emden, northern Germany, on 13 December and docked two days later in Kotka, southern Finland, to pick up a cargo of anchor chains, said Finnish Customs spokesman Petri Lounatmaa.
It was bound for the Chinese port of Shanghai but there was no indication for whom the military cargo was destined.
Routine checks by Finland's traffic safety authority revealed a load of up to 160 tonnes of improperly packed nitroguanidine, a low-sensitivity explosive with a high detonation speed.
"Actually in our investigation at the moment, we have got the information that we found 69 Patriot missiles on the ship and around 160 tonnes of explosives," said Detective Superintendent Timo Virtanen from the Finnish National Bureau of Investigation.
Interior Minister Rasanen said she had not heard of a similar case.
"Of course, there are legal transports of weapons or defence material [through Finland] but in this case the cargo was marked as containing fireworks," she told Finnish media. "That is quite unusual."
Mr Lounatmaa said customs officials and police had launched a joint investigation into a possible breach of Finnish export and weapons trading laws.
He said that the crew of about 32 were being questioned.
Patriot missiles, designed by the US company Raytheon, are supplied to "US and allied forces", according to the company's website. South Korea is among states which deploy them.