Russian border police have seized a Japanese fishing boat in waters near a chain of disputed islands, officials in Tokyo have said.
Japan and Russia both claim the seas around the disputed islands
A spokeswoman for the Japanese Coast Guard said she did not know what had happened to the six people on board.
Both Moscow and Tokyo claim the four islands, which Russia calls the southern Kurils and Japan the Northern Territories.
The row has stopped the nations signing a peace treaty officially ending WWII.
The islands were seized by the Soviet Union at the end of the war in 1945. Russia has said it will hand over some of the islands to Japan, but Tokyo insists all four should be returned.
The No 38 Zuisho Maru was captured on Sunday morning near Kunashiri, one of the four disputed islands, according to Japan Coast Guard spokeswoman Sanae Kobayashi.
She added that she did know where the crew were, or if they were safe.
Japan's Foreign Ministry has lodged a protest, demanding that Russia provide an explanation.
"Based on Japan's position on the Northern Territories issue, we cannot accept [the seizure] and request that the case [be] resolved promptly," the ministry said in a statement obtained by the Associated Press.
There have been several cases of Japanese boats being seized in the area in recent months, as Russian authorities have stepped up their patrols.
In August, a Russian coast guard boat opened fire on a Japanese fishing vessel, killing one of its four crew members.
Moscow accused the boat of poaching in its waters, and detained the vessel's captain for a month and a half.