Ady Gil in action.

Ady Gil in action.

Anti-whaling group Sea Shepherd's stealth boat Ady Gil was cut in half by a Japanese security vessel in the Antarctic today, says the group's leader Paul Watson.

The $1.5 million high tech vessel's wreckage was sinking but its crew of six were rescued and uninjured, he said.

Captain Watson said the Ady Gil was idling in waters near Commonwealth Bay when it was suddenly approached and rammed by the Japanese ship Shonan Maru, which has been detailed to provide security to the fleet.

Earlier today the fleet was contacted for the first time by the Ady Gil, and Sea Shepherd's surprise third vessel, the Bob Barker.

Captain Watson, aboard the Steve Irwin, said he was still 500 miles from collision point.

"This seriously escalates the whole situation," Captain Watson said of the collision.

According to the Institute of Cetacean Research, the Ady Gil's crew were launching projectiles at the Nisshin Maru, and attempted to entangle its propellers with rope.

Looking more like Batman's spacecraft than a boat, the biodiesel-powered trimaran Ady Gil arrived at the Southern Ocean stand-off between Japanese whalers and activists yesterday.

It was reported this morning to be about 50 nautical miles away from the whaling fleet.

Japan's Fisheries Minister Hirotaka Akamatsu had earlier warned two security ships would be used to protect the whaling fleet, intervening when Sea Shepherd tried to block the transfer of harpooned whales to the factory ship Nisshin Maru.

Captain Watson this morning announced the existence of a secret third ship in the Sea Shepherd flotilla.

He said he was confident that with three ships, Sea Shepherd would be able to cause maximum disruption to the whaling, which has been underway for around a month.

The Rudd Government has repeatedly called for caution by both sides in the wilds of the Antarctic.

"We have reminded the masters of protest vessels of their obligations under international law to take all steps to ensure safety of life at sea, particularly in the inhospitable conditions of the Southern Ocean," Environment Minister Peter Garrett said recently.

"We are also passing the same message to the government of Japan."