Environmentalists have clashed with the crew of Nisshin Maru before

The fleet is conducting its largest hunt in the South Pacific - it has instructions to kill up to 1,000 whales, including 50 humpbacks.

The humpback hunt is the first since a mid-1960s global ban and has drawn strong protests from environmentalists.

Commercial whaling was stopped in 1986 but Japan is permitted whaling in the name of scientific research.

Four whaling ships, including the lead craft Nisshin Maru, set off from the southern port of Shimonoseki on Sunday.

The 239-man mission plans to kill more than 900 minke whales as well as fin whales and humpbacks, in a South Pacific whale hunt that will run until mid-April.

The 8,000-metric ton Nisshin Maru was crippled by a fire on a whaling mission in the Antarctic in March. One crew member was killed.

A Greenpeace campaign ship will be following the Japanese fleet.

Sensitive mammals

Tokyo's plan to target the humpback - which was hunted to near extinction four decades ago - has drawn condemnation from environmentalists.

Crowds gathered to wave off the whaling boats

"Humpbacks are very sensitive and live in close-knit pods so even one death can be extremely damaging," Greenpeace spokesman Junichi Sato said.

Japanese fisheries officials insist both humpback and fin populations are back to sustainable levels.

"Humpback whales in our research area are rapidly recovering," said fisheries spokesman Hideki Moronuki.

"Taking 50 humpbacks from a population of tens of thousands will have no significant impact whatsoever."

Mr Moronuki said killing whales allowed marine biologists to study their internal organs.

Meat from Japan's scientific catch is sold commercially but Japanese officials deny that the mission plans to make a profit.

Tokyo argues that whaling is an ancient Japanese tradition, and has pushed unsuccessfully at the International Whaling Commission to reverse the 1986 moratorium on commercial whaling.

Environmentalists say Japan's research programme is a pretext for keeping the whaling industry alive.