A vessel operated by the Sea Shepherd Conservation Society on Friday attacked a Japanese ship conducting "research whaling" in the Antarctic Ocean by hurling about 15 bottles containing a foul-smelling liquid and powder, the Fisheries Agency said.
None of the 15 crew members on the Japanese ship was injured in the attack, which began at around 6 p.m. Japan time and continued for about three hours, according to the agency.
Sea Shepherd's ship, the Steve Irwin, bumped the 860-ton Kaiko Maru, but no major damage was sustained, agency officials said, adding that the incident will not affect its whaling activities.
The liquid in the bottles is believed to be butyric acid. The ingredients are being analyzed.
The Kaiko Maru has been hunting southern minke whales in the Antarctic Ocean with four other vessels.
The fleet, led by the Nisshin Maru, departed Japan in mid-November and plans to conduct research whaling in the Antarctic until mid-April next year.
The Sea Shepherd group has been repeatedly interfering with Japanese whalers since February last year.
Japan halted commercial whaling in 1986 in line with an international moratorium, but has been hunting whales since 1987 for what it calls scientific research purposes.
Environmentalists have condemned the loophole as a cover for commercial whaling.
Sea Shepherd was established in 1977 and is based in Washington state.