Japan's whaling fleet halted its Antarctic operations Wednesday and scrambled to arrange the turnover of two activists who boarded one of its harpoon ships after a tense, high-seas chase, accusing the Sea Shepherd conservation group of piracy.

The captain of the activists' ship meanwhile said the two were "hostages" and vowed to keep harassing the fleet.

"We will only accept an unconditional release," Sea Shepherd founder Paul Watson said by satellite phone from the antiwhaling ship Steve Irwin. He said he wanted action immediately.

"We're going to chase them until they stop their hunt," Watson said. "As long as we're chasing them, they're not killing any whales."

The Fisheries Agency said from Tokyo that it told the antiwhaling group to pick up the two activists, who boarded a harpoon ship Tuesday to deliver a protest letter. The agency said its only demand was that Sea Shepherd promise not to mount any attacks during the rendezvous.

Watson, however, said Japan was also demanding the group end its harassment of the whaling fleet as a condition for freeing the pair.

Glenn Inwood, a spokesman for Japan's Institute for Cetacean Research, which organizes the hunt, accused Sea Shepherd of stalling for publicity.

"It is completely illegal to board anyone's vessel . . . on the high seas," he said. "So this can be seen as nothing more than an act of piracy by the Sea Shepherd group."

Australia criticized both sides for behaving in a potentially dangerous way in a region that is thousands of kilometers from the nearest help in case of an emergency.

Australian Benjamin Potts, 28, and Briton Giles Lane, 35, boarded the Yushin Maru No. 2 after chasing it in an inflatable, and were tied up on deck while other Sea Shepherd activists threw bottles of acid onto the deck, Japanese officials said.

Watson said the activists wanted to deliver a letter demanding an end to the whale hunt and then leave, but were roughed up and held.

Australian Foreign Minister Stephen Smith said Australian officials had twice contacted their Japanese counterparts to press for the pair's swift release and were assured this would happen.

"From the very first day, I urged all parties in this matter to exercise restraint," Smith told ABC radio, referring to previously known plans for the environmentalists to chase the whalers. "It's quite clearly the case that restraint hasn't occurred here."

He warned later at a news conference: "We're dealing with the great distance of the Southern Ocean. The capacity for adverse incidents is high, and the capacity for rescue or assistance is low."

In Tokyo, Fisheries Agency official Takahide Naruko said the whalers decided to release the activists because they had done no damage to the ship, but noted Sea Shepherd had not responded to efforts to arrange the handover.

Hideki Moronuki, a spokesman for the Fisheries Agency's whaling section, said it sent safety instructions for the handover, but attached no further conditions in terms of Sea Shepherd's activities.

But Watson said Japan demanded that Sea Shepherd stop harassing, filming or photographing the whalers, and that the transfer of the pair be conducted by small boat, 10 nautical miles from the Yushin Maru.