Two Greenpeace ships Esperanza and Arctic Sunrise launched inflatable boats on Wednesday to harass Japanese "catcher boats", positioning them between the whale and harpoon gun.

Then several more "catcher boats", which had been whaling further away, returned with their load to the mother ship where the whales are cut up.

"So the Esperanza positioned itself immediately behind the mother ship so the catcher boats couldn't offload their whale carcasses. Some of the catcher boats tried to push the Esperanza out of the way by colliding with it," Greenpeace Australia Pacific Chief Executive Steve Shallhorn told Reuters on Thursday.

Japan abandoned commercial whaling in 1986 in line with an international moratorium and began what it calls a research programme the following year. Critics said the programme was a disguised commercial hunt for meat for upscale restaurants.

Despite international disapproval, Japan announced in June plans to nearly double its annual catch of minke whales to 850 and add fin whales and eventually humpbacks - two types of whales conservationists say whose survival is threatened.

"GET IN THE WAY"

Australia is a staunch critic of Japan's whaling programme and Prime Minister John Howard reiterated his opposition in a meeting with Japanese Prime Minister Junichiro Koizumi on the margins of a regional summit in Malaysia last week.

Howard warned both Greenpeace and the Japanese whalers on Thursday against any dangerous behaviour.

"I do not support action which endangers lives or breaks the law," he told reporters in Sydney.

The Japanese Fisheries Agency said it was the Greenpeace ship that had rammed the Japanese vessel and that it was considering filing criminal charges against some Greenpeace members who tried to make their way onto the Japanese ships.

"Our research whaling is in line with an international treaty and is perfectly legitimate," said Hideki Moronuki, head of the whaling section at the agency.

"We repeatedly announced this to the Greenpeace boats and sprayed water from our water cannons as a warning, but they ignored it all and rammed into our ship," he told Reuters.

Greenpeace said it planned to continue its action against the Japanese fleet for at least another month.

"As long we can maintain fuel supply in our vessels we think we can be down here for many, many weeks yet. The season goes right through until March and we are hoping to get in the way as much as we can," Greenpeace activist Shane Rattenbury told Australian radio via satellite phone from the Arctic Sunrise.