Activists from the Sea Shepherd Conservation Society, a US-based environmental group, have been chasing the Japanese fleet's mother ship. Their 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.

An official at the country's fisheries agency said whaling had been halted "for now" because of safety concerns. "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.

Commercial whaling was banned in 1986 but Japan uses a regulation permitting hunting for scientific research.

BBC