Anti-whaling activists and Japanese whalers exchanged water cannon fire in the Antarctic Ocean as two boats tailed the whaling fleet in an attempt to prevent the killing of any whales.

The Sea Shepherd conservation group said its ships, the Steve Irwin and the Bob Barker, had surprised the Japanese factory ship Nisshin Maru early yesterday.

The US-based activist group sends vessels to confront the Japanese fleet each year, and there have been two collisions already this year.

Commercial whaling is banned but Japan's hunt is allowed under world whaling regulations as a scientific expedition.