Finnish prosecutors filed negligence charges Thursday against the Ukrainian captain of a merchant ship found to have 69 Patriot missiles aboard, but lifted a travel ban on him and the first mate.

The charges relate to the improper loading of nearly 150 tons of explosives found aboard the vessel along with the missiles, and which officials said "posed a danger" to those on board.

The first mate, also Ukrainian, will be charged for his role in overseeing and approving the improper loading of the cargo.

"The charges mean that they have been summoned and may appear in court or be represented by an attorney... The case would most likely be heard in June," state prosecutor Juha-Mikko Haemaelaeinen told AFP.

The British-registered Thor Liberty has been at the southeastern Finnish port of Kotka since December 15, when the US-made surface-to-air missiles were found on board without the proper authorisation required to transit through Finnish territory.

The vessel was also found to be carrying 150 tons of nitroguanidine packed in cardboard boxes, rather than shipping containers.

Finnish officials launched an investigation into possible illegal transit of defence materiel, though a German defence ministry spokesman said soon afterwards that the missiles came from the German military and were destined for South Korea.

The spokesman said it was a "legal sale on the basis of an accord between two states at the government level" and that export authorisations were in order.

However a senior Finnish defence ministry official said Finland had not received a transit licence application for the missiles from Germany.

On Wednesday, the Finnish government approved the missiles' transit through Finnish territory to Seoul's Defence Acquisition Program Administration.

Finnish customs said Thursday they were nonetheless continuing their probe into the missiles.

"We will continue the investigation to try to find out what really happened, and whether anyone was negligent," the head of the Finnish customs anti-crime unit, Petri Lounatmaa, told AFP.

The removal of the travel ban and the granting of the transit permit clear the way for the ship to leave with the missiles on board.

It was not immediately known when the ship would depart. Finnish officials impounded the missiles during the investigation, and they will have to be reloaded on to the ship before it can depart.