Japan has ordered the release of two activists detained when they boarded one of its harpoon vessels in Antarctica to deliver an anti-whaling protest, Australian Foreign Minister Stephen Smith said Wednesday.

Paul Watson, the captain of the Steve Irwin protest ship, said the conditions set for the release of Australia Benjamin Potts, 28, and British national Giles Lane, 35, were unacceptable.

"They are saying we have to agree to not take any action against the whaling activities, not to video or photo their whaling activities, ... which I'm not going to do," Watson told Australia's AAP news agency.

Watson alleged the pair were assaulted and held above decks on the Yushin Maru 2 for two and a half-hours in freezing weather before being taken below.

"When you hold hostages and make demands, that's the definition of a terrorist organization and that's the way they are acting," Watson charged. "We're not going to cease and desist from interfering in their illegal whaling activities as a condition."

Smith said neither captain should set conditions on the transfer of Potts and Lane.

"Japan has given us assurances that the two will be released and returned to their ship," Smith said.

The five-vessel fleet left Japan in November with the intention of returning with 935 minke whales and 50 fin whales. Initially, 50 humpbacks were to be taken as well, but this part of the catch is in abeyance after Australia and New Zealand led an international protest.

Glenn Inwood - spokesman for the Institute of Cetacean Research, a Japanese whaling organization - said the pair had been tied up to restrain them after a failed attempt to foul the propeller. He described the accusation that they had been assaulted as "absolute lies."

The new Australian government led by Prime Minister Kevin Rudd has been accused of talking tough on Japan's annual whale hunt but doing little to stop it.

An Australian ship, the Oceanic Viking, left Perth last week on a 20-day mission to monitor the Japanese ships.

Watson called the Oceanic Viking a "ghost ship" because it hadn't approached the Japanese despite being given the coordinates of the fleet's location by the protestors.