Japan has suspended its annual Antarctic whale hunt following protests from a campaign group.
Activists from the Sea Shepherd Conservation Society, a US-based environmental group, have been chasing the Japanese fleet's mother ship.
An official at the country's fisheries agency said whaling had been halted "for now" because of safety concerns.
Commercial whaling was banned in 1986 but Japan uses a regulation permitting hunting for scientific research.
Iceland and Norway have lodged official objections to the ban and continue to hunt commercially.
Japan says it suspended its hunt on 10 February. It is unclear whether the expedition, which would usually end mid-March, will be called off permanently.
"Putting safety as a priority, the fleet has halted scientific whaling for now. We are currently considering what to do hereafter," Tatsuya Nakaoku, an official at the fisheries agency, told Reuters news agency.
But he said nothing had been decided yet.
Activists' ships have been harrying the fleet for weeks in the icy seas of Antarctica.
The Sea Shepherd Conservation Society says one of its boats has been blocking the main ship's stern loading ramp, preventing any harpooned whales from being loaded on to the ship.
According to the environmentalists, in recent days the whalers have left their usual hunting ground off Antarctica and are now heading towards the southern tip of South America.
"If that's true then it demonstrates that our tactics, our strategies, have been successful," Sea Shepherd captain Paul Watson told the AFP news agency by satellite phone from the Steve Irwin ship.
"I don't think they've gotten more than 30 whales... certainly they haven't got many whales at all."
Japan's fleet involves 180 people on four ships, with the aim to kill up to 945 whales in Antarctic waters during the southern winter season.
Japan says it continues to hunt for scientific research, while not concealing the fact that much of the meat ends up on dinner plates, the BBC's Roland Buerk in Tokyo reports.
Few Japanese eat whale regularly, but many object to what they see as unjustified foreign interference in a cultural tradition, our correspondent adds.
Anti-whaling nations, led by Australia and New Zealand, and environmental groups say the hunts are cruel and unnecessary. Australia is taking legal action in the International Court of Justice against Tokyo over whaling.