Militant anti-whaling activists yesterday 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 yesterday to deliver a protest letter.