TOKYO (Reuters) - Japan on Wednesday demanded the prompt release of the crews of five fishing boats detained by Russia, saying they were doing nothing wrong and calling for them to be set free on humanitarian grounds.

The seizure of the Japanese vessels the day before by Russian border guards near Russian-held islands claimed by Japan came while Japanese Foreign Minister Toshimitsu Motegi is visiting Russia seeking to restart stalled negotiations over the islands and a formal peace treaty to end World War Two.

The ships were fishing for octopus near the four islands when they were taken by Russian ships for “investigations” on suspicion they had violated fishing agreements, Chief Cabinet Secretary Yoshihide Suga told a news conference on Wednesday.

“This is unacceptable,” he said, adding the fishing crews had done nothing wrong.

“The government is strongly demanding, from a humanitarian perspective, the early release of the crew and ships.”

A Japanese crab fishing boat was seized by Russia in late January this year, with its crew released a bit over a month later after paying a fine.

Japan claims the four Russian-held islands off Japan’s northern region of Hokkaido, which it calls the Northern Territories. Known in Russia as the Southern Kuriles, the islands were invaded by the then-Soviet army in the waning days of World War Two.

The islands row has prevented Russia and Japan from signing a formal peace treaty, but Russian President Vladimir Putin suggested to Japanese Prime Minister Shinzo Abe last year that they sign an accord by year-end “without any pre-conditions”.

Although Abe rejected that proposal, he vowed in January to push for a treaty. Progress has proved elusive, although senior officials and political leaders from both countries have since held talks on the issue.

Japanese media has reported that Motegi’s current five-day visit to Moscow is aiming to restart negotiations.