Militant anti-whaling activists have threatened commando-style raids against a Japanese whaling boat holding two colleagues as Australia offered to help end a two-day confrontation in icy seas near Antarctica.

"We will try and do everything we can to stop them killing whales. That's what we are doing down here," Paul Watson, captain of the anti-whaling protest ship Steve Irwin named after the late television conservationist, told local media.

Whaling has halted in the Southern Ocean after two anti-whaling activists were detained after scrambling aboard a Japanese whaling boat on Tuesday to deliver a protest letter. Both sides have accused the other of behaving like terrorists and the Japanese have given Watson's Sea Shepherd Conservation Society a list of conditions for the return of Australian Benjamin Potts and Briton Giles Lane.

Australia's Foreign Minister Stephen Smith, in contact with Japan's Prime Minister Yasuo Fukuda to avoid a diplomatic rift, said he had asked an Australian patrol ship to pick up the men as a neutral intermediary. "We would like the transfer to be expedited as soon as possible, but people should understand it is a difficult operation," Smith said.

The customs icebreaker Oceanic Viking was in sight of the Japanese whaling fleet and trying to contact both Watson and the the captain of the Japanese ship Yushin Maru No.2, he said. Steve Irwin's widow, Terri Irwin, on Thursday backed the radical tactics employed by Sea Shepherd and Watson, who last year collided with a Japanese whale hunter.

"I'm very proud of Paul Watson for doing something so actively positive to save whales," Irwin told Sydney's The Daily Telegraph newspaper. Surveys showed most Australians thought the group had gone too far in Tuesday's protest. Japanese whaling officials have said they are prepared to released the two activists to a Sea Shepherd inflatable boat if the Steve Irwin protest ship stays 10 nautical miles clear of the whaler Yushin Maru No.2 during the transfer. "We want to hand them over as soon as possible and have offered to do so, but there has been no response from Sea Shepherd," Chief Cabinet Secretary Nobutaka Machimura said.

Watson rejected any conditions as "extortion" and said he may launch more protests if he is allowed to tie his ship directly to the Japanese whaler for a transfer. "We board poaching vessels all the time," he said, threatening a commando-style raid on the Japanese ship by other activists if the standoff was not resolved.

"It would be an act of desperation, but I'm not going to let them take them back to Japan," Watson said. A spokesman for the Japanese Institute of Cetacean Research said the skipper of the whaling boat feared more protests if Watson came alongside and the two activists may have to be taken to Japan if Australia could not end the standoff.

"They have to realise we're not just going to tie up the Yushin Maru to the Steve Irwin vessel," Glenn Inwood said. "There are security risks associated with that," he said.

Australia's Prime Minister Kevin Rudd called for calm on all sides to avoid any accidents in frigid and treacherous seas. "It's important for all parties to exercise restraint when it comes to the possible danger to a human life," Rudd said.

The Oceanic Viking is seeking to gather photo and video evidence for an international legal challenge against Tokyo's "scientific" whaling. Japan plans to hunt almost 1,000 minke and fin whales for research over the Antarctic summer, but has abandoned the cull of 50 humpback whales after international condemnation and a formal diplomatic protest by 31 nations.

Despite a moratorium on whaling, Japan is allowed an annual "scientific" hunt, arguing whaling is a cherished cultural tradition and the hunt is necessary to study whales. Its fleet has killed 7,000 Antarctic minkes over the past 20 years.

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