Japan has sent a giant military icebreaker to bolster its whaling fleet in the conflict with Sea Shepherd off the Australian Antarctic Territory, anti-whaling activists say.

The 12,500 tonne Shirase, operated by the Japan Maritime Self-Defence Force, has appeared near whalers and Sea Shepherd activists 50 nautical miles off the coast of the territory, the group said.

The ship was recognised on radar by its identification signal, according to Sea Shepherd.

The Korean tanker Sun Laurel was making a renewed attempt to refuel the whalers' factory ship Nisshin Maru under the shelter of a Coast Guard helicopter from Shirase, Sea Shepherd founder Paul Watson said.