Japan's whaling authorities said Friday they are suing militant environmentalist group Sea Shepherd and its head in the United States in a bid to stop them interfering in the annual whale hunt.

It is the first time that Japan has attempted legal action abroad against anti-whaling campaigners, who have sometimes used extreme methods against ships involved in the hunt, carried out under rules that allow research whaling.

"Today, Kyodo Senpaku Kaisha and the Institute of Cetacean Research along with research vessels' masters filed a lawsuit against the Sea Shepherd Conservation Society (SSCS) and Paul Watson," they said in a statement.

"The Institute of Cetacean Research and Kyodo Senpaku are seeking a court order in the US District Court in Seattle, Washington that prevents SSCS and its founder Paul Watson from engaging in activities at sea that could cause injuries to the crews and damage to the vessels."

Kyodo Senpaku owns ships, while the cetacean institute operates the whaling programme under the authority of the Japanese government.

Sea Shepherd, based in Washington state, regularly sends vessels to harass the whalers. In previous years it has thrown stink bombs onto the decks of the Japanese fleet, while vessels from both sides have repeatedly clashed.

The Japanese statement said the whaling programme was "greatly contributing to the advancement of scientific knowledge of whale resources in the Antarctic".

Commercial whaling was banned under a 1986 International Whaling Commission agreement. "Lethal research" is allowed, but other nations and environmental groups like Sea Shepherd condemn it as disguised commercial whaling.

Tokyo says the whale hunts are needed to substantiate its view that there is a robust whale population in the world. However, it makes no secret of the fact that whale meat from this research ends up on dinner tables and in restaurants.

The statement condemned Sea Shepherd's actions as "life-threatening".

"Sabotage activities against the research fleet by SSCS and Paul Watson have been escalating over several years," it said.

"The activities perpetrated by SSCS and Paul Watson not only put at risk the safety of the research vessels at sea but are also affecting the scientific achievement" of the programme, it said.

In February, Japan cut short its hunt for the 2010-2011 season by one month after bagging only one fifth of its planned catch, blaming interference from Sea Shepherd.

Sea Shepherd founder Paul Watson said he was not worried about the lawsuit, claiming the activists were not aggressive and did not break any laws.

"We are not down there protesting whaling, we are down there intervening against criminal activities," he told Australian news agency AAP from onboard Sea Shepherd's ship the Steve Irwin off Fremantle, Western Australia.

"We defend ourselves from being rammed, hit with water cannons, shot at, have concussion grenades and bamboo spears thrown at us, so yes, we defend ourselves."

Watson -- who travels to Australia each year to lead activists in their efforts against the whale hunt -- said that the ships his organisation used were not owned by Sea Shepherd USA, nor were they US-flagged vessels.

"The United States government and courts have no authority over these ships so I don't know what they are hoping to achieve," he said.

The Japanese legal action came after the whaling fleet left port Tuesday for this season's annual hunt.

The coast guard has deployed an unspecified number of guards to protect the ships from anti-whaling activists, and the Japanese government has confirmed it will use some of the public funds earmarked for reconstruction after the massive March earthquake and tsunami to boost security for the hunt.

Three ships from the Sea Shepherd fleet are due to set sail over the coming days to once again confront the Japanese whalers, the organisation said.

The Steve Irwin and the Brigitte Bardot will leave from Albany in Western Australia and the Bob Barker will depart from Hobart in Tasmania.