Finnish authorities have detained two Ukrainians over 69 surface-to-air Patriot missiles found on board a ship that docked in Finland en route to China, customs officials said Thursday.

"The ship's captain and the first mate have been detained," the head of the Finnish customs anti-crime unit, Petri Lounatmaa, told AFP.

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

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 Finnish port of Kotka about 120 kilometres (75 miles) from Helsinki.

A German defence ministry spokesman said the Patriot missiles came from the German military and were destined for South Korea.

He said it was a "legal sale on the basis of an accord between two states at the government level". He said the transaction had received an official export authorisation and was reported to customs authorities.

Finnish police said Wednesday the ship's destination was the Chinese port of Shanghai.

Lounatmaa said the Thor Liberty's first officers and crew of about 30 were all Ukrainians, and that interrogations were under way.

He said investigators would be looking more closely into the intended destination of the vessel and its cargo, which also included propellant charges for the missiles, and 150 tons of explosives.

Port officials have relocated the vessel to a separate berth at the Kotka port.

Finnish customs have confiscated the missiles, and "the Finnish military are taking care of their transportation and storage," Lounatmaa said.

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