The Sea Shepherd anti-whaling group said three Australian activists were being held as "prisoners" by the Japanese harpoon fleet Sunday after sneaking aboard one of their vessels overnight to protest. Sea Shepherd said they had helped the three men from the Forest Rescue Australia environmental group to board the Shonan Maru No.2 16 miles (26 kilometres) off Australia's west coast. "The boats approached the Shonan Maru under the cover of darkness and the three negotiated their way past the razor wire and spikes and over the rails to successfully board the Japanese whaling vessel," Sea Shepherd said of the Forest Rescue trio. They were now "being held as prisoners on board the Japanese vessel the Shonan Maru No. 2" which had been tailing the Sea Shepherd ship the Steve Irwin and contained "armed Japanese military personnel," Sea Shepherd claimed. In statement Forest Rescue said it wanted to prevent the Shonan Maru from following the Steve Irwin back to the Southern Ocean. The Steve Irwin had to return to Australia last week because another Sea Shepherd ship, the Brigitte Bardot, was damaged in high seas and had to be escorted home for repairs, setting back the group's annual hunt of the whalers. Forest Rescue said it was "making a stand to assist Sea Shepherd Conservation Society in their campaign to end illegal whale poaching in the Southern Ocean Whale Sanctuary." "We are taking this action to remind the Australian government of their obligation to enforce existing laws pertaining to the prohibition of whaling ships in our waters," the group added. "Forest Rescue are insulted and disappointed in our government for allowing the transit of whale poaching vessels in Australian waters." The group has demanded that the Japanese return the three men -- Simon Peterffy, 44, Geoffrey Tuxworth, 47 and Glen Pendlebury, 27 -- to shore and leave Australian waters. Commercial whaling is banned under an international treaty but Japan has since 1987 used a loophole to carry out "lethal research" in the name of science -- a practice condemned by environmentalists and anti-whaling nations. Australia has taken Japan to the International Court of Justice seeking an end to its annual hunts but one of the men who boarded the Shonan Maru, Simon Peterffy, said enough had not been done. "We are on board this ship because our government has failed to uphold its pre-election promise to end whaling in the Southern Ocean," he said, according to the Forest Rescue statement.