Photo/Illutration The Northern Territories off Cape Nosappu in Nemuro, Hokkaido (Asahi Shimbun file photo)

Russian security authorities seized five Japanese fishing boats and their crews in waters off the disputed Northern Territories on Dec. 17 and towed them to Kunashiri island for an inspection of the cargoes.

Japan's Foreign Ministry demanded through diplomatic channels that Russia immediately return the vessels and crew.

Russia claimed it was fully justified in making the seizure, according to sources close to the Foreign Ministry.

The boats belong to Hokkaido fishery associations in Nemuro, Ochiishi and Habomai, according to authorities in Hokkaido.

Three other Japanese fishing boats operating in the area were left undisturbed.

Officials in Hokkaido told the Fisheries Agency that the crew of the five seized vessels are unharmed and in good health.

After being stopped by a Russian border security patrol, the vessels were taken to Furukamappu on Kunashiri Island, known in Russia as Yuzhno-Kurilsk.

The Japanese vessels were fishing for octopus under an agreement on operations in those waters between Japan and Russia, the Foreign Ministry sources said.

The Northern Territories off the eastern coast of Hokkaido were seized by the Soviet Union at the end of World War II. Japan has spent decades trying to regain sovereignty.

A 1998 agreement between the two nations on fishing rights in the area allows Japan to catch octopus, Atka mackerel and Alaska pollack in return for fees.