Greenpeace said Monday it has disrupted the Japanese whale hunt off Antarctica by chasing the fleet's whale processing factory ship out of the whaling zone.

The six-vessel fleet "scattered and ran" early Saturday when it realized the Greenpeace vessel Esperanza was "heading toward them at high speed," Greenpeace expedition leader Karli Thomas told New Zealand's National Radio.

The fleet's three whale hunter vessels "can't operate without the [factory ship] Nisshin Maru there to process the kill," she added.

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Greenpeace has pledged to take nonviolent action to try to stop the ships from killing whales, which in the past has led to activists in speed boats trying to put themselves between whales and Japanese harpoons, and once led to a ship collision.

A spokesman for Japan's whale hunt called Greenpeace's actions illegal and demanded it stop its disruptive actions.

"Greenpeace actions are illegal under international law [and] it's time the public stopped treating Greenpeace as heroes," Glenn Inwood, spokesman for the Institute of Cetacean Research, in Tokyo, Japan, said Monday. "It's time the public saw this fringe group for what they really are: environmental imperialists who are trying to dictate their morals to the world."

Japan dispatched its whaling fleet to the icy waters of Antarctica in November to kill about 1,000 whales under a program that Tokyo says is for scientific purposes, but which anti-whaling nations and activists say is a front for commercial whaling.

Under worldwide pressure, Japan last month abandoned its plan to include 50 humpback whales in this season's hunt — the first major hunt of humpback whales since the 1960s. It still plans to kill 935 minke whales and 50 fin whales.