"If our crew can hit them, then they would be better off quitting the research vessel and joining a professional baseball team," Shigeki Takaya, an assistant director of the Far Seas Fisheries Division at Japan's fisheries ministry, said.

Sea Shepherd, which Japan accuses of "eco-terrorism" for its repeated attempts to disrupt the annual whale hunt, said the whaling fleet had used a new "acoustic weapon" against the activists.

"The factory ship the Nisshin Maru and the two harpoon vessels in the fleet are equipped with long range acoustical devices," it said in a statement.

"This is a military grade weapon system that sends out mid to high frequency sound waves designed to disorient and possibly incapacitate personnel. It is basically an anti-personnel weapons system."

Sea confrontation

Paul Watson, a spokesman for the anti-whaling campaign, said the Steve Irwin retreated when within range of the acoustic weapon but generally had "a very successful day".

"All we need to do is to keep them running and to keep them from whaling and that is exactly what we are doing."

There was a similar confrontation during last year's whale hunt [GALLO/GETTY/SSCS]
In Tokyo, the Fisheries Agency said the activists had hurled projectiles and tried to attach themselves with a rope to one of the vessels, prompting the whalers to respond with water sprays and "beeping warning tones".

Japan has asked the embassies in Australia and New Zealand as well as the Netherlands –where Steve Irwin is registered – to prevent a similar incident, said a fisheries official.

"Their ship is still following the whaling fleet," Shigeki Takaya told the AFP news agency.

Glenn Inwood, a local spokesman for Japan's government-backed Institute of Cetacean Research, said the whalers would not reveal their tactics "to protect themselves from the criminal actions committed by the Dutch vessel".

"We can say, however, that all legal means available will be used to ensure these pirates do not board Japanese ships or threaten the lives of the crews or the safety of the vessels," Inwood told Australia's national AAP news agency.

An international moratorium on commercial whaling was imposed in 1986 but Japan kills hundreds each year using a loophole that allows "lethal research".

Japan makes no secret of the fact that the meat ends up on dinner tables and accuses Western nations of not respecting its culture.