A radical environmental group has agreed to pay $2.55 million to Japanese whalers for breaching a US court injunction to stay clear of their vessels in the Antarctic Ocean.

The United States-based Sea Shepherd Conservation Society and former senior officials of the group last week agreed to pay the sum to resolve civil contempt charges against them in the US Ninth Circuit Court of Appeals, court documents showed.

The payment will go to the Institute of Cetacean Research and Kyodo Senpaku, the two main bodies behind Japan's so-called "research" whaling programme.

The Ninth Circuit's injunction prohibited Sea Shepherd founder Paul Watson and any person acting with them from physically attacking the vessels operated by the Japanese entities in the Southern Ocean or acting in a manner to endanger their navigation.

The preliminary injunction, issued in late 2012, also bans the defendants from coming within 500 yards (457 metres) of the vessels.

But Sea Shepherd boats were involved in high-seas confrontations with the whalers in early 2013.

Japanese authorities have previously described methods used by Sea Shepherd -- such as blocking the Japanese ships' propellers -- as "terrorism".

In December 2014, the US court issued an opinion finding Sea Shepherd, Watson and others in "contempt of the injunction".

- 'Scientific research' -

Japan's Institute of Cetacean Research (ICR) hailed the payment, saying in a statement that it hopes the move would lead to the "prevention of unreasonable hindrance to our research" for now and in the future.

Sea Shepherd legal counsel Claire Loebs Davis said in a statement that the group does not agree with the Ninth Circuit's holding that it was in contempt.

"But after more than two years of litigation, we are very pleased to be putting the contempt action behind us," she said.

Davis added that the group would now focus on other continuing litigation, which she said would give it an opportunity to expose the ICR's "dangerous and illegal activities" in the Southern Ocean.

The commercial hunting of whales is prohibited in the Southern Ocean Whale Sanctuary, but Japan has hunted the animals there using a "scientific research" loophole.

It has never made any secret of the fact that meat from the whales caught in the course of this research makes its way onto menus.

Japan was forced to call off the 2014-2015 hunt after the United Nations' top court, the International Court of Justice, ruled last year that its annual mission to the Antarctic was a commercial hunt masquerading as science to skirt the international ban.

Tokyo maintains it is seeking scientific answers to questions about the size and health of the ocean's whale population, for which it has to slaughter the animals and examine the contents of their stomachs, or the condition of their internal organs.

Critics say nothing very scientific has ever come from these missions, which have been carried out for nearly three decades. They say any properly-designed research programme would by now know the answers it is purporting to seek.