An anti-whaling group said Sunday that three activists are being held on a Japanese whaling ship after sneaking aboard the vessel overnight.

The three activists are from the Australian group Forest Rescue and were assisted by the Sea Shepherd Conservation Society, whose ships are tailing the Japanese whaling fleet as it heads towards the Southern Ocean.

Sea Shepherd says the three Australian activists have not been returned and describes them as “prisoners” on the Shonan Maru 2.

A Sea Shepherd activist boarded the same ship in 2010 and spent five months behind bars in Japan before being convicted on a variety of charges and deported.

Commercial whaling is banned under an international treaty, but Japan continues to hunt using a loophole that allows whaling in the name of science, a practice condemned by environmentalists and anti-whaling nations.