Tokyo says Russia has seized a Japanese whaling patrol vessel after it allegedly entered the country's territorial waters.
Russia said the ship intruded into its waters in the Sea of Okhotsk between Kamchatka and the Russian mainland.
The ship has 19 Japanese crew and one Russian observer on board.
ITAR-TASS quotes the Japanese state fishery agency as saying the vessel crossed the Russian border by mistake.
Japan's Kyodo news agency said the boat has been held since August 15. It said Russian authorities have inspected the ship and questioned the captain and crew.
The ship, which does not catch whales itself, was said to have been conducting a "visual check" of whales in the Sea of Okhotsk.
Ties between Russia and Japan have soured as Tokyo has joined its Western allies in imposing sanctions on Russia in the wake of its annexation of Crimea.