Anti-whaling activists Sea Shepherd on Tuesday said Japan's fleet had been separated and driven out of its Antarctic hunting ground, declaring an early success in its campaign to disrupt the annual hunt.
The militant group said the five-vessel fleet was "in disarray" and currently not hunting whales, with the harpoon ships separated from each other by hundreds of miles and the factory ship Nisshin Maru well outside its kill zone.
"The Nisshin Maru is on the run and unable to stop and whale in its self-designated whale-poaching grounds," Sea Shepherd said in a statement.
Sea Shepherd has three ships on the high seas to disrupt the harpooners and on Monday said it had located all five Japanese vessels, releasing photo evidence that four whales had been killed.
Since then the Japanese fleet has split up and their operations cut short, the activist group said.
"Should the Nisshin Maru attempt to return to the whaling grounds, Sea Shepherd will be ready to once again intercept and shut down their illegal whaling operations," it said.
The commercial hunting of whales is prohibited in the Southern Ocean Whale Sanctuary, which was designated by the International Whaling Commission in 1994, but Japan catches the animals there under a "scientific research" loophole in the moratorium on whaling.
Australia has taken Japan to the International Court of Justice seeking to have its research whaling programme declared illegal, with a ruling due this year.
Siddarth Chakravarty, captain of one of the Sea Shepherd vessels the Steve Irwin, said it had been an encouraging start to the group's 10th annual harassment campaign, which got under way last month.
"Within a day-and-a-half we have the entire whaling fleet in disarray," he said.
High-seas clashes between the two groups are common, resulting in the 2010 sinking of the Sea Shepherd vessel Ady Gil.