The environmentalist group Sea Shepherd has accused Japanese whalers of ramming and sinking one of their boats, the high tech Ady Gil, in the Antarctic Ocean.

Sea Shepherd said the powerboat was cut in half by Japanese security ship Shonan Maru No. 2 as it loitered near the whaling fleet. Whalers said the Ady Gil was launching projectiles and that the activists were trying to tangle propellers.

All six crew of the Ady Gil were rescued but the almost one-million-euro trimaran is sinking. The environmentalist group believes the action was deliberate.

Tensions over the whole whaling issue are mounting as Australia's government comes under pressure by parliamentarians to block so-called "spy flights" launched by Japan from Australian airports to foil activists.

The Ady Gil, a 24-metre carbon-fibre trimaran that runs on low-emission fuels, was the latest addition to the Sea Shepherd protest fleet. The materials and paint on the boat made it difficult for radars to detect, enabling it to sneak up on whaling vessels and disrupt the hunt.