Japan has been stepping up international pressure to try to rein in the Sea Shepherd Conservation Society, which has vowed to physically stop the slaughter of the ocean giants.

Sea Shepherd said its ship, the Steve Irwin, which collided last week with a whaling vessel, was heading back to Australia with only four days of fuel reserves left.

"Another four days is simply not worth getting someone killed," said Paul Watson, the Canadian captain of the ship.

"We have done everything we could with the resources available to us this year," he said in a statement. "We have cost them money and we have saved the lives of a good many whales."

He vowed to return next season -- and said he hoped to come with a faster ship to hassle the whaling fleet.

"I intend to be their on-going nightmare every year until they stop their horrific and unlawful slaughter of the great whales in the Southern Ocean Whale Sanctuary."

Japan, which says whaling is part of its culture, hunts up to 850 whales each year in the Antarctic Ocean despite strong objections from political allies Australia and New Zealand.

But for the previous two seasons Japan's catch was curbed largely because of harassment by environmentalists.

Japan kills whales using a loophole in a 1986 international moratorium on commercial whaling that allows "lethal research" on the mammals, and makes no secret of the fact that the animals' meat is then served as food.

Only Norway and Iceland defy the whaling moratorium altogether.

Japan last week summoned the ambassador of The Netherlands, where the Steve Irwin is registered, to demand it take action against the environmentalists.

Asked if the diplomatic protest led to Sea Shepherd's decision, Toshinori Uoya of the Fisheries Agency told AFP: "As we have repeatedly said, their activity is illegal and unforgivable."

"They have to be punished based on international law and The Netherlands, as its nationality is on the ship, bears primary responsibility for the crackdown," Uoya said.

He added that the whalers would maintain security precautions despite Sea Shepherd's withdrawal.

"We don't know why they are stopping their attacks against us this season, but we don't trust anything they say. They may return to attack us again," he said.

Japan has complained after activists hurled bottles of rancid butter at the whalers and tried to board the ship.

Sea Shepherd in turn accused Japan of crossing the line by deploying acoustic weapons, which send out high-frequency sound waves to disorient the activists.

The environmentalists said that use of the sonic weapons left three of their crew with injuries, with one man requiring five stitches above his left eye.

Unlike Sea Shepherd, the more mainstream environmental group Greenpeace this season held off from chasing the whalers.

Greenpeace said it wanted to focus on defending two of its activists who are set to go on trial on charges of stealing whale meat as part of a bid to expose alleged corruption involving whalers.

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