Environmental campaigners from the militant Sea Shepherd group said Wednesday a Japanese whaling ship had rammed its vessels in their worst confrontation in the Southern Ocean in three years.
Sea Shepherd said the Japanese factory boat the Nisshin Maru had deliberately collided with boats crewed by anti-whaling campaigners, the Steve Irwin and the Bob Barker -- allegations denied by Japan.
"There has been the most outrageous attack on the Sea Shepherd Australia ships today; multiple rammings of those ships by the giant factory whaling ship from Japan," said co-campaign leader Bob Brown.
"Water cannon laid on the ships and concussion grenades lobbed on the ships from the Japanese government escort vessel.
"Japan through its loud hailers... ordered Australian ships out of Australian territorial waters while they wanted to proceed with their illegal refuelling of the Nisshin Maru... to allow them to continue their illegal whaling in this international and Australian whale sanctuary."
Brown said the incident had destroyed a lifeboat on the South Korean tanker the Sun Laurel, which was refuelling the Japanese factory ship when the clash occurred.
He said it was the worst confrontation since a January 2010 collision that caused the campaigners' vessel the Ady Gil to sink, with the Bob Barker sandwiched between the Nisshin Maru and the Sun Laurel at one point.
Sea Shepherd said the Nisshin Maru rammed the Bob Barker several times, including from behind, destroying one of their radars and all of their masts and causing it to lose all power and issue a MayDay distress call.
Power has since been restored to the ship.
"It's extraordinarily dangerous and it's just a direct break of international law," said Brown, the former leader of Australia's Greens party.
"Canberra needs to tell Tokyo to haul off immediately and to dispatch naval vessels down there to uphold international law."
Sea Shepherd's Jeff Hansen said the Steve Irwin had been struck on the stern and right hull.
"We're now under attack with concussion grenades being thrown by armed Japanese coastguard in Australian waters at our crew," he told ABC television.
"We're very concerned for the safety of our crew at the moment."
Japan's Fisheries Agency gave a different version of events, saying three Sea Shepherd boats rammed the Nisshin Maru at least four times when the Japanese ship was about to be refuelled by a tanker.
In a statement it said the Nisshin Maru's bow and a railing was damaged.
The incident comes after the US Supreme Court earlier this month denied a plea from Sea Shepherd to throw out a lower court's injunction to stay at least 500 yards away from the whaling vessels.
The Sea Shepherd Conservation Society is chasing the Japanese fleet hunting whales off Antarctica, as it has done for years in a bid to harass the whalers and prevent the mammals being slaughtered.
Environment Minister Tony Burke has described Japan's whale hunt as cruel and unnecessary but has so far rejected calls to send an Australian government vessel to monitor the hunt.
He said he was unable to confirm details of the alleged clash.
"The government condemns so-called 'scientific' whaling in all waters and we urge everyone in the ocean to observe safety at sea," he said.
Australia strongly opposes whaling and launched legal action challenging the basis of Japan's so-called "scientific" hunt in December 2010.
Japan claims it conducts vital scientific research using a loophole in an international ban on whaling, but makes no secret of the fact that the mammals ultimately end up on dinner plates.