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

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

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