Graphic footage shows dead whales lying on the deck of a Japanese fishing vessel that was allegedly operating in an internationally recognised whale sanctuary.

Anti-whaling group Sea Shepherd shot video from the air showing 'Nisshin Maru' sailing through the Southern Ocean Whale Sanctuary carrying whale carcasses.

According to the group, a fleet of five Japanese vessels was spotted inside the sanctuary and crew had used grenade-tipped harpoons to capture four whales which were then slaughtered. Sea Shepherd (SS) claimed that the boats began to flee the sanctuary after being spotted from the air.

"That's just a gruesome, bloody, medieval scene which has no place in this modern world," said SS chairman, Bob Brown. "Sea Shepherd will do all it peaceably can to prevent this grotesque and cruel destruction."

Blood on the decks of Japanese whaling boat captured in Southern Ocean Whale Sanctuary by Sea Shepherd
Blood on the decks of Japanese whaling boat captured in Southern Ocean Whale Sanctuary by Sea Shepherd Sea Shepherd

The sanctuary lies in Antarctica's Ross Dependency near New Zealand's territorial waters. The slaughter was condemned as "a gross breach of international law" by the campaign group.

Japan said the catches were for scientific purposes, which is permitted under International Whaling Commission (IWC) rules. The foreign minister of New Zealand claimed that no whaling activity had taken place in its waters.

A spokesman for the fisheries agency of Japan denied the existence of a whaling sanctuary, as established by the IWC, adding that so-called "research whaling" was "not a violation or an abuse of a loophole in the convention".

"Quite the contrary, this is a legitimate right of the contracting party under Article VIII of the International Convention for the Regulation of Whaling."

Sea Shepherd chairman Bob Brown said: "There's three carcasses on the ship, a fourth carcass has been cut up. There's blood all over the place, meat being carted around on this factory ship deck, offal and innards being dumped in the ocean. There is nothing scientific about this, it is butchery."

He called for international action against Japan by Australia and New Zealand. "The one thing that's missing here is gumption - a bit of spine in Canberra and in Wellington to put an end to it."

Australiawants the "research whaling programme" clause scrapped by the International Court of Justice. An Australian military jet is due to carry out surveillance in the Southern Ocean for 12 weeks from this month.