Japanese whalers are holding two anti-whaling activists who leapt aboard a Japanese ship near Antarctica. The Japanese fleet is being followed by environmental campaigners, who have promised to disrupt the annual whaling expedition. From Sydney, Phil Mercer reports.

Two members of the anti-whaling group Sea Shepherd boarded the Japanese vessel Yushin Maru No. 2 Tuesday. Their colleagues say they were trying to deliver a letter demanding an end to the whale hunt.

The two - Australian Benjamin Pott and Giles Lane of Britain - were detained by crew members. The Japanese say the activists illegally boarded the ship, and attacked crewmen with bottles of acid.

A Sea Shepherd spokesman says the two men have been mistreated, and called their detention a kidnapping.

Glenn Inwood, a spokesman for the whaling fleet, says there is a more appropriate word for the two men's actions.

"It is completely illegal to board anyone's vessel, any other country'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. And this is exactly the reason why we have called for these groups to back off."

The Sea Shepherd group has been shadowing the Japanese fleet in Antarctic waters aboard its own boat, named "The Steve Irwin" after the famous Australian conservationist who died in 2006.

Paul Watson, the captain of the Sea Shepherd boat, says the Japanese have offered to hand the two men back. But Watson says Japanese conditions for doing so are unacceptable.

"...There was a condition on us getting the men back, and that condition was that we would refrain from harassing and interfering with their whaling operations, and that's certainly a condition we're not going to meet," said Watson.

The confrontation is a dramatic escalation of the annual contest between the whaling fleet and environmental groups that try to stop them. The international group Greenpeace has also sent a boat to harass the Japanese fleet, and earlier this week claimed to have chased two Japanese ships out of the whale hunting grounds.

The Australian government has urged both sides to exercise restraint, and has called on Tokyo to push for the release of the two campaigners.

Japan sent its ships to Antarctica in November to kill about a thousand whales. The Japanese annually circumvent an international ban on whaling by classifying the hunt as "scientific research."

Australia, which has vigorously opposed the hunt, has described it as an unnecessary slaughter.