Sea Shepherd said the substance they threw at the whalers was harmless
Japanese officials have reacted angrily to an attack on one of its whaling ships by Sea Shepherd anti-whaling activists in the Antarctic.
Fisheries Minister Hirotaka Akamatsu said the group threw butyric acid - made from rancid butter - at the ship, mildly injuring three crew members.
But Sea Shepherd said no injuries were reported in the clash.
Japan has six whaling vessels in the Antarctic, where whalers and activists regularly clash during the hunt season.
The latest clash reportedly lasted for several hours and involved two Sea Shepherd vessels and four whaling ships.
Japan said three Japanese crew members suffered mild face and eye injuries from the acid.
Mr Akamatsu said he was glad the whalers had not been seriously hurt in the incident but that he was "filled with great anger" over the incident.
In a statement on its website, Sea Shepherd said it had fired warning flares when three of the whalers attempted to destroy one of its helicopters with a water cannon.
THE LEGALITIES OF WHALING
Objection - A country formally objects to the International Whaling Commission (IWC) moratorium, declaring itself exempt. Example: Norway
Scientific - A nation issues unilateral 'scientific permits'; any IWC member can do this. Example: Japan
Aboriginal - IWC grants permits to indigenous groups for subsistence food. Example: Alaskan Inupiat
The group said it then launched a small boat from its ship, the Steve Irwin, and "annoyed the harpoon vessels with rotten butter bomb attacks".
It said the substance, which it has used on previous occasions, was unpleasant but harmless.
Sea Shepherd founder Paul Watson said the group had prevented any whaling from taking place for the past week.
"Our goal now is to make it two weeks and then three weeks. We will not tolerate the death of a single whale," he said.
The clash comes a week after Sea Shepherd said its ship Bob Barker had been "intentionally rammed" and damaged by a whaler.
In January, the group said its hi-tech speedboat the Ady Gil had been severely damaged after being rammed by a whaler.
Japan abandoned commercial whaling in 1986 after agreeing to a global moratorium; but international rules allow it to continue hunting under the auspices of a research programme.
It says the annual hunt catches mostly minke whales, which are not an endangered species.
Conservationists say the whaling is a cover for the sale and consumption of whale meat.