The Greenpeace ship Esperanza was running low on fuel and needed to turn back, expedition leader Karli Thomas said in a statement.

"While the Esperanza must return to port, the campaign to stop whaling in the Southern Ocean Whale Sanctuary is far from over," Thomas said.

"The pressure we have created on the high seas must now be translated into action by governments around the world, the Japanese people themselves and Japanese companies".

The Greenpeace vessel, along with a ship owned by the more militant environmental group the Sea Shepherd Conservation Society, has been following the Japanese fleet for two weeks.

Both groups claim to have chased Japanese boats out of the hunt zone, frustrating the whalers' plans to kill more than 1,000 whales this year.

Japan exploits a loophole in a 1986 moratorium on commercial whaling which allows hunting for research purposes.

In the statement, Greenpeace said the Esperanza had been on a 4,300 nautical mile chase of the fleet's factory ship, the Nisshin Maru, since spotting the vessel in the early hours of January 12.

"Without the factory ship, the remaining hunter vessels have been unable to operate -- bringing the entire whaling programme to a halt," it said.

Australia has led protests against the whale kill and has sent a customs boat to the Southern Ocean to document the whalers' activities to potentially pursue an international case against them.

Japan argues that whaling is part of its culture and has frequently accused Western nations of cultural insensitivity.

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