Japanese whalers returned to port Thursday after an Antarctic hunt that killed more than 300 of the mammals, the government said.

The fleet had set sail for the Southern Ocean in December, with plans to slaughter 333 minke whales, despite a worldwide moratorium and opposition led by Australia and New Zealand.

Japan's Fisheries Agency announced Thursday that the target number of "scientific research" kills had been achieved.

The 2015/16 season came after a one-year hiatus prompted by a ruling by the United Nations' International Court of Justice (ICJ), which said the annual hunt was a commercial venture masquerading as science.

Under the International Whaling Commission, to which Japan is a signatory, there has been a moratorium on hunting whales since 1986.

But Japan persists in the practice using a loophole in the ban that allows for lethal 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 whale meat ends up on dinner tables and is served up in school lunches.

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

However, the government of Prime Minister Shinzo Abe had been determined to resume the hunt.

- 'Unnecessary' -

The haul announced Thursday is bigger than it has been in recent years when the mission has been hampered by a confrontational campaign on the high seas by environmentalist group Sea Shepherd.

The group, which attracts support from celebrities including actress Brigitte Bardot, has harangued Japanese vessels in the Southern Ocean, and has claimed success in drastically reducing the catch.

In the 2013-14 season, just 251 minke whales were caught, while the figure was only 103 in the season before.

That compares with historic catches of around 850.

The returning fleet arrived early Thursday at Shimonoseki port in western Japan, the fisheries agency said.

Besides the kills, the agency also said it conducted non-lethal research such as observation, the taking of skin samples from live whales and attaching tracking devices to whales.

"Attaching GPS devices helps us study minke whales' migration routes by tracking them for several days," agency official Hiroyuki Morita told AFP.

Environmental campaign group Greenpeace on Thursday labelled the hunt "unnecessary" and said it violated the UN court ruling.

"It is completely unacceptable for the Japanese government to ignore the ICJ's findings and furthermore, completely unnecessary to go ahead with lethal research," said Greenpeace Japan executive director Junichi Sato.

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.

Some experts say that 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.