US conservation group Sea Shepherd vowed Tuesday to fight a US court order to stay at least 500 yards away from Japanese whaling ships, and to keep protecting whales "with our ships and our lives."

The injunction was ordered by the US Court of Appeals for the Ninth Circuit, in the latest step in a legal battle between the anti-whaling group and Japanese authorities over vessels in the Southern Ocean.

It said Sea Shepherd and Canadian militant conservationist Paul Watson, who is wanted by Interpol, "are enjoined from physically attacking any vessel engaged by plaintiffs," including Japan's Institute of Cetacean Research.

In addition, they are banned from "navigating in a manner that is likely to endanger the safe navigation of any such vessel," said the order, issued on Monday.

"In no event shall defendants approach plaintiffs any closer than 457.2 meters when defendants are navigating on the open sea," it added. The joint plaintiffs are Kyodo Senpaku Kaisha, Ltd., Tomoyuki Ogawa and Toshiyuki Miura.

In a statement, the Institute of Cetacean Research and Kyodo Senpaku said they "welcome" the injunction, which remains in force until the US court issues its opinion on the currently pending appeal.

The groups said they had asked the court to expedite its review because Sea Shepherd had "launched its sabotage vessels and announced its intention once again to take physical action against the Japanese research vessels."

But Charles Moure, an attorney for Oregon-based Sea Shepherd, told AFP in an email that the court injunction was "very disappointing," adding: "We intend to fight the order."

And in a statement on its website, the group called the US court ruling "the first shot of the season" by Japanese whalers.

"It is a complex situation whereby a United States Court is issuing an injunction against Dutch and Australian vessels carrying an international crew, operating out of Australia and New Zealand in international waters," it said.

"In addition the Court has ignored the fact that the Japanese whalers are in contempt of a court order by the Australian Federal Court and the whaling takes place in the Southern Ocean Whale Sanctuary."

And it vowed to continue to protect whales in the Southern Ocean, saying that Japan's fleet "will find when they arrive that we will still be there guarding the Southern Ocean Whale Sanctuary with our ships and our lives."

"We will defend these whales as we have for the last eight years -- non-violently and legally," said Watson, quoted in the statement.

Confrontations between the whalers and activists have escalated in recent years, and the Japanese cut their hunt short in early 2011 due to Sea Shepherd harassment.

Japan hunts whales using a loophole in a global moratorium that allows killing the sea mammals for what it calls "scientific research," although the meat is later sold openly in shops and restaurants.

Watson confirmed this month that he is back onboard a Sea Shepherd vessel and ready to confront Japanese whalers.

His whereabouts had been a mystery since July, when he jumped bail in Germany, where he was arrested on charges stemming from a high-seas confrontation over shark finning in 2002.

Sea Shepherd's ninth campaign, named Operation Zero Tolerance, is its largest ever against Japan's whale hunt and involves four ships, a helicopter, three drones and more than 100 crew members.

Three of the vessels, the Steve Irwin, Bob Barker and Brigitte Bardot, are all at sea while the Sam Simon is at an undisclosed location.