(Adds Australia Greens comment, quotes)
By Rob Taylor
CANBERRA, Jan 9 (Reuters) - Australia urged Japan on Monday to release three anti-whaling activists detained while boarding a Japanese whaling fleet security ship and said that the vessel, now sailing in Australian economic waters, was not welcome.
The three Australian men, who illegally boarded the whaling support vessel Shonan Maru 2 in darkness off the coast of Western Australia state on Sunday, were subject to Japanese laws because they had been detained on the high seas, Australia's Attorney-General Nicola Roxon said on Monday.
But the presence of the Japanese security ship threatened to stoke tensions over the yearly Japanese whale cull in the frigid Southern Ocean, Roxon said, and could worsen clashes with the anti-whaling activist group Sea Shepherd, which has targeted the fleet for the last eight years.
"We've made very clear that this boat is not a welcome boat in our Exclusive Economic Zone (EEZ)," Roxon told Australian television, hinting the government could order action against the Japanese ship if it strayed further into Australian waters.
"This ship, people need to remember, is not directly involved in whaling activities, but it is clearly providing a support role and that may give us some other options if it was trying to come into our territorial waters," she said.
The three activists from the Australian group Forest Rescue boarded the ship with assistance from Sea Shepherd Conservation Society, whose ships are trying to tail the Japanese whaling fleet as it heads towards the Southern Ocean.
The trio, named as Simon Peterffy, 44, Geoffrey Tuxworth, 47 and Glen Pendlebury, 27, carried a message reading: "Return us to shore in Australia and then remove yourself from our waters."
Forest Rescue is an environmental group which specialises in direct action, usually to prevent the logging of forests.
Roxon said the men could be taken to Japan to face charges as the boarding had occurred outside Australian territorial waters and inside the EEZ, which gives Canberra sovereign rights only over the area's natural resources.
But she said she hoped that would not occur and a deal could be done with Japan -- Australia's second-biggest trade partner and a member with the United States of a three-nation security pact -- to transfer them to Australian authorities.
"We are representing our views most strongly that they should be released promptly and returned to Australian soil," Roxon said.
In 2008 an Australian, Benjamin Potts, and another Sea Shepherd member who had boarded a Japanese whaler in the Southern Ocean were handed over to the Australian Customs vessel Oceanic Viking near Antartica.
Australia's politically influential Greens party said it would try to bring a bill into parliament forcing Canberra to send a patrol ship to monitor the whale hunt, something the government has refused to do this year to avoid damaging diplomatic ties with Tokyo.
"These three Australians are heroes for the whales. They've come from a forest protest background from trying to protect the biggest living entities on the planet's land to trying to protect the biggest living creatures in the ocean," Greens leader Bob Brown told reporters.
But Donald Rothwell, an international law expert from the Australian National University, said the three activists could face a wide range of charges under Japanese law and may even have broken Australian law.
"Unauthorised boarding of a Japanese vessel is an act of trespass wherever that act may have taken place at sea," Rothwell said.
Last season, Japan cut short its annual whale hunt with less than a fifth of their quota in response to Sea Shepherd harassment which saw an activist boat -- The Ady Gil -- sunk in a collision with a Japanese ship.
Japan -- which along with Iceland and Norway is one of one three countries that hunt whales -- introduced scientific whaling to skirt the commercial whaling ban under a 1986 moratorium. It argues it has a right to monitor the whales' impact on its fishing industry.
Last year, Australia filed a complaint against Japan at the world court in the Hague to stop Southern Ocean scientific whaling. A decision is expected in 2013 or later.
Glen Inwood, a New Zealand-based spokesman for Japan's Institute of Cetacean Research, told Australian radio that the three men could be stuck on the Shonan Maru 2 for as long as the ship was shadowing Sea Shepherd, possibly until March or April.
"Not only are they facing that, but they certainly risk being taken to Japan to be tried for trespassing or whatevever other charges that Japan feels they may want to issue against them," Inwood said. (Additional reporting by Sonali Paul in MELBOURNE; Editing by Ed Davies)