SYDNEY (AFP) - Militant anti-whaling campaigners Sea Shepherd said on Monday they were ambushed under the cover of darkness by a Japanese whaling fleet in a "ruthless assault" to disable their ship in the Southern Ocean.

Sea Shepherd said two Japanese harpoon ships attacked their Bob Barker vessel on Sunday night, using steel cables in an attempt to jam its propellers and rudders from close range in violation of laws aimed at preventing collisions at sea.

Bob Barker captain Peter Hammarstedt said he had been tailing the Japanese factory ship Nisshin Maru since Sunday morning but the harpooners waited until night fell - "launching their onslaught under the cover of darkness".

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 marks the second such showdown in a month.

"We maintain unwavering dedication in the face of an increasingly vicious onslaught from the poachers, the last line of defence standing between these criminals and the protected whales they are targeting to kill," Mr Hammarstedt said.

The captain said he radioed the Japanese ships to inform them that his operations were "completely lawful and that any aggressive action on their part would be reported to the Australian government and New Zealand Search and Rescue."

"I have nine Australians and one New Zealander on board as part of my brave crew who have endured this ruthless assault," he said.

Sea Shepherd Australia director Jeff Hansen said both Australia and New Zealand had challenged Japan's whaling programme at the International Court of Justice.

"So where are these governments now, when their citizens... are under attack from these poachers?" he asked.

During the incident Sea Shepherd said its small boats crews were pelted with ice by the Japanese, who also flashed the Bob Barker's bridge with blinding searchlights to allow the Nisshin Maru to escape.

The factory ship was now being tracked by Sea Shepherd's two other vessels, the Sam Simon and the Steve Irwin, while the Bob Barker continued to shadow the harpoon ships, claiming they were currently "unable to whale".

It is the second time the two groups have come to blows this month, with Sea Shepherd claiming the Bob Barker was struck during a coordinated attack from the Japanese a few weeks ago.

Tokyo confirmed that collision but accused Sea Shepherd of throwing rope at its ships.

In 2010 a collision resulted in the sinking of Sea Shepherd's speedboat Ady Gil.