Russia has seized a Japanese research whaling ship in the Sea of Okhotsk on suspicion of intruding into Russian territorial waters, government sources said on Friday.

The 712-ton Shonan Maru No. 2 left Japan on Aug. 8 to examine whales in the Sea of Okhotsk at the request of the government-linked Fisheries Research Agency.

Russian authorities say the patrol ship failed to notify them of a change in its planned route. The research was originally set to take place in Russia's exclusive economic zone but the ship intruded into Russian territorial waters, the sources said.

The skipper and crew members of the ship have been questioned by Russian authorities since Aug. 15 on a voluntary basis while the Shonan Maru No. 2 sits anchored at a Russian port, said the agency, which is part of the Agriculture, Forestry and Fisheries Ministry.

The ship used to accompany Japan's research whaling fleet on trips to the Southern Ocean but has been conducting a visual survey of whales in accordance with plans worked out by the International Whaling Commission.

The skipper is believed to have refused to admit the ship intruded into Russian territorial waters, the sources added.