In this photo released by the Sea Shepherd Conservation Society, its crew members are hosed by water cannons from the Japanese harpoon whaling ship, the Yushin Maru No. 1, as the Sea Shepherd helicopter flies alongside on Thursday in the Ross Sea off Antarctica. (Sea Shepherd Conservation Society/Steve Roest/Associated Press)

A boat carrying a group of radical anti-whaling activists collided with a Japanese whaling vessel in the Antarctic Ocean on Friday in a clash Japan condemned as "unforgivable." No one was injured.

Activist Paul Watson of the Canadian-founded Sea Shepherd Conservation Society said his boat was trying to prevent a Japanese ship from dragging a whale on board when another Japanese boat shot in front of Watson's vessel, causing a collision.

"The situation down here is getting very, very chaotic and very aggressive," Watson told the Associated Press by satellite phone from his vessel the Steve Irwin, named after the late Australian conservationist and television personality.

In a statement, the Institute of Cetacean Research — the Japanese government-affiliated organization that oversees the hunt — condemned the protesters' actions, characterizing the collision as a "deliberate ramming" that occurred while the Japanese were trying to load a whale on board one of their ships.

Shigeki Takaya, a Fisheries Agency spokesman for whaling in Japan, called the incident "appalling and unforgivable."

"We will ask concerning countries, including Australia, to immediately stop them from carrying out such horrendous acts," Takaya said.

Japan plans to harvest up to 935 minke whales and 50 fin whales this season. Under International Whaling Commission rules, the mammals may be killed for research but not for commercial purposes. Opponents say the Japanese research expeditions are simply a cover for commercial whaling, which was banned outright in 1986.

Chiharu Tsuruoka, a Foreign Ministry official in Japan, called Friday's incident between Japan's whalers and the Dutch-registered Steve Irwin "extremely unforgivable."

"We have repeatedly asked the Dutch government to stop them from harassing us, but so far it's been so unsuccessful," Tsuruoka said.

Familiar situation for Sea Shepherd

Friday's collision isn't the first such incident for the Sea Shepherd group, which is known for its high-profile, confrontational tactics. In 2007, a Sea Shepherd ship collided with a Japanese whale-spotting vessel in the Antarctic seas.

Japanese officials accused the group of attacking the whaling ship like pirates, while Watson said at the time that the whaling ship hit the Sea Shepherd ship twice, tearing a gash in its hull.

The group's website lists its efforts against whalers, including "scuttling half of the Icelandic whaling fleet and whale processing station" and "scuttling of the Norwegian whaling vessels Nybraena and Senet."

"We intend to be more aggressive and even more relentless in our interventions," Sea Shepherd member Peter Hammarstedt is quoted as saying on the group's website.

"We do not intend to witness the killing of any whales; we intend to make sure no whales are killed on our watch."

The protesters set off from Australia in early December for the remote and icy Antarctic Ocean, chasing the whaling fleet for about 3,200 kilometres before stopping two weeks ago in Tasmania to refuel. The group found the whalers again on Sunday and resumed their pursuit.