Three Australian environmental activists boarded a Japanese whaling vessel Sunday to protest against Japan's annual whale cull in the Antarctic, anti-whaling group Sea Shepherd said in a statement.

The three activists from the Australian group Forest Rescue boarded the ship early Sunday with assistance from Sea Shepherd Conservation Society, whose ships are trying to tail the Japanese whaling fleet as it heads toward the Southern Ocean.

The activists had not been returned and were prisoners now detained on a Japanese whaler, Sea Shepherd said.

Forest Rescue is an environmental group which specialises in direct action, usually to prevent the logging of forests.

A group spokesman, Michael Montgomery, confirmed the incident and said the action was to demand the departure of the Japanese whalers from Australian waters.

We don't need to kill these beautiful creatures anymore, he told Reuters.

Sea Shepherd said the activists came from shore in a boat, which approached the vessel Shonan Maru 2 in the dark, with assistance from two Sea Shepherd boats.

The three negotiated their way past the razor wire and spikes and over the rails of the Japanese whaling vessel, the group said in its statement.

They are being held in Australian territorial waters by an invading Japanese vessel containing armed Japanese military personnel.

They carried with them a message reading: Return us to shore in Australia and then remove yourself from our waters.

Whaling was banned under a 1986 moratorium, but Japan continues to hunt hundreds of whales annually under a loophole that allows whaling for scientific purposes.

The Institute of Cetacean Research, the Japanese body that coordinates the hunt, had no immediate comment on its website about the latest incident.

(Editing by Robert Birsel)