Japan has demanded Russia release 14 crew members of a fishing boat seized last week, a top Japanese official said Thursday, asserting that the ship was operating legally.

The Eiho Maru No.172 was seized by Russia between Japan's northern region and Sakhalin which is the Russian Far East.

Moscow has said it was operating in Russia's economic waters, but Japanese government spokesman Katsunobu Kato denied that claim and demanded the fishermen be freed.

"From yesterday through today, we have used the diplomatic channel... to protest and to communicate that Russia's pursuit, search, taking and detention of the ship is unacceptable," Kato told a regular briefing.

"We have demanded the crew members and the vessel be released immediately," he said, adding that Tokyo's analysis showed the ship was operating inside Japan's economic waters.

The captured crew members are in good health and have enough food and water, Kato said.

Japan and Russia routinely dispute over fishing rights.