Australia's federal government is under pressure to take swift action to secure the release of three Australian activists detained aboard a Japanese whaling security ship.
But the government has moved to lower expectations of what it can do and has conceded the men could be taken to Japan to face legal action.
The West Australian men from the Forest Rescue environmental group boarded the Japanese security vessel Shonan Maru No 2 in waters off the coast of Bunbury overnight.
The daring mission was aimed at forcing the vessel to stop tailing the Sea Shepherd Conservation Society's anti-whaling flagship, the Steve Irwin.
The men climbed past razorwire and spikes to board the ship and deliver a message: "Return us to shore in Australia and then remove yourself from our waters."
But the Shonan Maru No 2 on Sunday afternoon was instead persisting with its pursuit of the Steve Irwin, with the Australians still on board.
A spokesman for the Japanese whaling program at the Institute of Cetacean Research, Glenn Inwood, confirmed the men were still aboard the vessel.
"They are unhurt, they are being questioned and there has been no decision on anything beyond that at this stage," the New Zealand-based Mr Inwood told AAP.
Mr Inwood said the men had boarded the vessel well outside Australian territorial waters - about 40km from the coast - and any suggestion otherwise was false.
The Sea Shepherd and Forest Rescue say the incident happened 16.2 miles off the coast - outside territorial waters but inside Australia's 24-mile contiguous zone.
Attorney-General Nicola Roxon said Australia's embassy in Tokyo had contacted the Japanese government seeking more information, most pressingly the vessel's location.
"If this vessel is close to Australian waters you'd think there was a possibility that (they) would promptly have discussions with us about a safe and immediate return," Ms Roxon told reporters in Melbourne.
"There are a range of different options that could occur for the transfer of these people back into Australian control."
But Ms Roxon expressed no confidence this would happen, saying the government's options were restricted because the incident happened outside territorial waters.
"It is likely that these three Australians may be taken back to Japan," she said.
Sea Shepherd head captain Paul Watson urged the government to take swift action to prevent that happening.
"I think the Australian government would be very embarrassed if an armed Japanese vessel can just pick up Australian citizens in Australia and then take them away to Japan," Captain Watson told AAP from the Steve Irwin.
"I think it's their obligation to protect the interests of their own citizens.
"Japanese vessels have no right to take prisoners in Australian waters."
Opposition environment spokesman Greg Hunt said the government had ignored repeated warnings this sort of incident might happen.
"They should immediately dispatch a Customs vessel," Mr Hunt told AAP.
"They should also seek the immediate transfer of the prisoners from the Japanese ship to Australian authorities."
The Australian Greens called on Prime Minister Julia Gillard to personally intervene by contacting her Japanese counterpart.
"The protesters have acted as a direct result of our own government's failure to take the necessary steps to end illegal whaling in the Southern Ocean," Greens Senator Rachel Siewert said.
She said it would be "unacceptable" for the ship to return to the Southern Ocean with the protesters still on board.
The three activists have been named as Geoffrey Owen Tuxworth, 47, of Perth, Simon Peterffy, 44, of Bunbury, and Glen Pendlebury, 27, of Fremantle.
Forest Rescue issued a statement saying they had "come from the forests of Australia" to defend the whales.
"We are on board this ship because our government has failed to uphold its pre-election promise to end whaling in the Southern Ocean," Mr Peterffy was quoted as saying.
The Steve Irwin was in Australian waters to deliver fellow anti-whaling ship Brigitte Bardot to port for repairs after it was damaged by a freak wave last month.