Five Japanese boats were fishing for octopus near disputed Russian-held islands when they were seized.

Japan‘s foreign ministry demanded the release on Wednesday of the crew of five fishing boats detained by Russia, saying the men had been doing nothing wrong and should be released quickly on humanitarian grounds.

The Japanese were seized by border guards from Russia near disputed islands north of Hokkaido that are held by Russia and claimed by Japan, as Japanese Foreign Minister Toshimitsu Motegi was visiting Russia.

The ships were fishing for octopus near the four islands – known as the Southern Kurils in Russia and Northern Territories in Japan – 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.

The fishing crews were detained as Japanese Foreign Minister Toshimitsu Motegi (right) was in Russia for talks [File: Franck Robichon/AFP]

“This is unacceptable,” Suga 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 just over a month later after paying a fine.

The disputed islands were invaded by the then-Soviet army in the waning days of World War II Two.

The islands dispute 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 promised 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 aimed at restarting negotiations.