An appeals court in the United States has labelled the anti-whaling group Sea Shepherd as pirates, clearing the way for Japanese whalers to pursue legal action against it.

In making his ruling, chief judge Alex Kozinski said people no longer needed a peg leg or an eye patch to be classified as pirates.

"When you ram ships, hurl glass containers of acid, drag metal-reinforced ropes in the water to damage propellers and rudders, launch smoke bombs and flares with hooks; and point high-powered lasers at other ships, you are, without a doubt, a pirate," he said.

This was true "no matter how high-minded you believe your purpose to be," the judge added, in a ruling that dubbed Sea Shepherd founder Paul Watson "eccentric."

Sea Shepherd is chasing the Japanese fleet hunting whales off Antarctica, as it has done for years in a bid to prevent the mammals being slaughtered.

Japan's Institute of Cetacean Research and others are pursuing legal action in the United States, seeking an injunction against Sea Shepherd's activities on the high seas.

The court ruling overturned a US district judge's ruling and allowed the Institute of Cetacean Research to pursue their action against the anti-whaling group.

The plaintiffs "are Japanese researchers who hunt whales in the Southern Ocean," which is regulated by an international convention, of which the United States and Japan are signatories, it noted.

The convention "authorises whale hunting when conducted in compliance with a research permit issued by a signatory," said the ruling.

"Cetacean has such a permit from Japan. Nonetheless, it has been hounded on the high seas for years by a group calling itself Sea Shepherd Conservation Society and its eccentric founder, Paul Watson.

"The activities that Cetacean alleges Sea Shepherd has engaged in are clear instances of violent acts for private ends, the very embodiment of piracy. The district court erred in dismissing Cetacean's piracy claims."

The appeals court also ruled the case should be transferred to another district judge.

"The district judge's numerous, serious and obvious errors identified in our opinion raise doubts as to whether he will be perceived as impartial in presiding over this high-profile case," it said.

"The appearance of justice would be served if the case were transferred to another district judge, drawn at random."

ABC/AFP