A Japanese whaling fleet set sail for the Antarctic on Tuesday, on a mission to resume the slaughter after a one-year pause, with environmentalists slamming the move as a "crime against nature".

Government officials and families of crew members stood on the quayside and waved as ships -- at least one fitted with a powerful harpoon -- left a southern port, television footage showed.

"Two whaling ships departed from Shimonoseki with a Fisheries Agency patrol boat this morning, while the factory ship also left another port to form a fleet," an agency official told AFP.

"A fourth whaler already left a northeastern port yesterday to join the fleet."

Despite a worldwide moratorium and opposition from usually-friendly nations like Australia and New Zealand, Japan persists in hunting whales for what it says is scientific research.

Tokyo claims it is trying to prove the whale population is large enough to sustain a return to commercial hunting, and says it has to kill the mammals to carry out its research properly.

However, it makes no secret of the fact that the animals' meat ends up on the dinner table or served up in school lunches.

In 2014, the United Nations' highest court, the International Court of Justice (ICJ), ruled that Japan's annual Southern Ocean expedition was a commercial hunt masquerading as science to skirt the moratorium.

In response, Japan's 2014-15 mission carried out only "non-lethal research" such as taking skin samples and doing headcounts.

But the government has said for months it intended to resume butchery in the current season, which runs to around the end of March.

The announcement Monday that the hunt was to begin drew condemnation from around the world.

Claire Bass, executive director for Humane Society International, said Japan had chosen to ignore the "universal opposition" represented by the ICJ ruling.

"Once again we have Japan's whaling fleet setting sail to commit a crime against nature," she said in a statement, stressing "Japan's long history of whale persecution".

Other conservationists called for another legal challenge.

The International Fund for Animal Welfare and the Australian Marine Conservation Society said a panel of legal experts asked to consider Japan's latest whaling mission had found it broke international law.

"The panel concluded that Japan's new whaling programme violates international law and that Australia or other countries still have options to challenge Japan's actions before international courts," said chair and Australian National University professor Donald Rothwell.

Japan has hunted whales for centuries, and their meat was a key source of protein in the immediate post-World War II years when the country was desperately poor.

But consumption has dramatically declined in recent decades, with significant proportions of the population saying they "never" or "rarely" eat whale meat.

Atsushi Ishii, an expert on international relations at Japan's Tohoku University, said Japan's refusal to give up the Antarctic mission despite censure by the international court is largely due to a small group of powerful politicians.

"Why resume whaling? Because a group of pro-whaling lawmakers don't like the image that they succumbed to pressure from Sea Shepherd," he told AFP, referring to an environmental group that has repeatedly clashed with Japanese whaling missions.

Sea Shepherd Australia said Monday it would follow the latest mission, which Japan said would aim to kill a total of 333 minke whales -- some two-thirds under previous targets.

Tokyo said in response that it would try to secure the safety of the 160 crew members by sending patrol boats to guard the fleet and strengthening "self-protection measures."

"The arguments made by Japan and by anti-whaling countries never meet halfway because they are talking about two different goals under the same rules," Katsuaki Morita, a professor at Konan Women's University and an expert on whaling history, told AFP.

"Anti-whaling countries see the IWC as the organisation for conservation, while Japan sees it as the body for ensuring sustainable commercial whaling under appropriate controls."