Japan was caught killing a whale deep inside Antarctic waters declared by Australia to be a protected whale sanctuary.

Sea Shepherd activists tracked the Japanese whaling fleet on Sunday, capturing the first images of Japan hunting whales in the Southern Ocean since Tokyo defied an international court ruling that declared the hunt to be illegal.

But the activists have since suffered a setback in their hope to disrupt the whale slaughter when heavy fog set in overnight and they lost track of the main abattoir ship, Nisshin Maru. The hunt was several hundred miles from Casey Station in Antarctica.

One of the Japanese harpoon ships sought to delay the Sea Shepherd vessel Steve Irwin, allowing the slower Nisshin Maru to escape radar range.