Three Australian activists who climbed aboard a Japanese whaling ship early Sunday have been detained and could be taken to Japan to face legal action.

A Canadian "eco-warrior" who helped the trio jump aboard the vessel off Australia's southwestern coast is calling for swift action to secure the men's release and stop whale hunting.

"It's just really unacceptable…(Japan) is getting away with poaching," Paul Watson, founder of the Sea Shepherd Conservation Society, told Â鶹´«Ã½.

Early Sunday, Sea Shepherd's flagship vessel, the Steve Irwin, approached the Japanese Shonan Maru No. 2 and helped three members of the environmental group Forest Rescue climb aboard using two smaller boats. The Japanese vessel had been trailing Watson's Sea Shepherd.

Australians Geoffrey Owen Tuxworth, 47, Simon Peterffy, 44, and Glen Pendlebury, 27, were detained and face possible prosecution in Japan.

In a statement, Forest Rescue said it's trying to end "illegal poaching" of whales in Antarctic waters. The U.S.-based Sea Shepherd society mostly works in international waters, sometimes joining forces with other activist groups.

The Australian government told local media it's been in touch with the Japanese government to make sure its citizens are safe. The fate of the activists is still unknown.

The whale hunts, which Japan says are for scientific purposes, are allowed by the International Whaling Commission as an exception to the 1986 ban on whaling. But opponents say they are a cover for commercial whaling because whale meat not used for study is sold for consumption in Japan.

"These people are criminals," Watson said of the Japanese whaling crew.

Glenn Inwood, spokesman for Japan's Tokyo-based Institute of Cetacean Research, which sponsors the annual whale hunt, confirmed the three activists were being questioned aboard the ship. They are not injured, he said.

"I would describe them as volunteer detainees," Inwood said.

Inwood told The Sydney Morning Herald the men had boarded the vessel well outside Australian territorial waters, about 40 kilometres from the coast. But Sea Shepherd and Forest Rescue claim the incident happened 16.2 nautical miles off the coast - outside territorial waters but inside Australia's 24-mile contiguous zone, the newspaper reports.

The Japanese vessel is a former harpoon boat that now serves as security for the whaling fleet. The fact that three men were able to climb past the ship's razor wire and spikes is an "embarrassment for the Japanese," Watson said.

His ship will continue tailing the Shonan Maru No. 2 until the situation is resolved.

"They haven't killed a single whale yet and we're trying to keep it that way," Watson said.

The Sea Shepherd's mission to end the destruction of habitat and wildlife in international waters has "incredible support" from Australians, but not the Australian government, Watson said.

Watson, a controversial figure who has been called a spotlight-hogging "eco-warrior" in some media profiles, founded Sea Shepherd in 1977 after he was forced out of Greenpeace.

In the 1980s, he protested against the Canadian seal hunt and used his ship to block ports in Newfoundland. Watson's wildlife preservation campaigns have been supported by many celebrities over the years, including Brigitte Bardot, Darryl Hannah and Mick Jagger.

With files from The Associated Press