Environmental activist group Sea Shepherd claimed that Japanese whalers had openly attacked its vessels in the Southern Ocean.

The group accused the Australian government of breaking promises to monitor whaling operations.

Sea Shepherd said Japanese harpoon ships Yushin Maru and Yushin Maru 3 had towed steel cables across the bow of its vessel the Bob Barker 11 times on Sunday in a bid to jam its propeller and rudder.

It said that when the Bob Barker launched two small boats to defend the ship and cut the steel cables, a bamboo spear was thrown at crew members.

Japan's fisheries agency claimed that its ships were dragging cables with buoys as a warning to Sea Shepherd to stay away.

"The cables were already there and they approached them on their own," an agency official said, denying knowledge of a spear being thrown.

"It was designed for them not to come near the Japanese ship."

High-seas confrontations are common between Sea Shepherd and the Japanese, who hunt whales off Antarctica under a "scientific research" loophole in the moratorium on whaling.

Sunday's incident was the third clash since the whaling season started earlier this year.

"Each time we have located the Nisshin Maru, the Sea Shepherd fleet has been attacked by the whalers in night-time ambushes," said Bob Barker captain Peter Hammarstedt.

The ship's helicopter located the Nisshin Maru early on Sunday with a minke whale onboard and "slabs of whale meat were photographed on the deck," the group said.