A Briton and Australian working for a conservationist group have been "taken hostage" by Japanese whalers, campaigners say.

The Sea Shepherd Conservation Society said the two volunteers were tied to the railings and radar mast of a whaling ship after they boarded the vessel.

The organization is chasing the whaling fleet across the Southern Ocean and is urging Australian police to file kidnapping charges.

• Click here to visit FOXNews.com's Natural Science Center.

"Our attempts to radio the vessels go unanswered," Sea Shepherd spokeswoman Christine Vasic told Sky News Online. "Our ships are still in pursuit and we have filed reports with the British and Australian consulates asking that they demand the release of our crew members."

"The Steve Irwin has dispatched a small fast Delta boat and a helicopter to attempt to persuade the Yushin Maru No. 2 to stop and release the hostages," she added.

Briton Giles Lane, 35, and Australian Benjamin Potts 28, had boarded the Yushin Maru to tell the whalers they were operating "in violation of international conservation law by targeting endangered species in an established whale sanctuary", the society said.

Japan's Institute for Cetacean Research denied the two men had been tied up.

"Any accusations that we have tied them up or assaulted them are completely untrue," the Institute's director-general, Minoru Morimoto, said in a statement posted on its Web site. "It is illegal to board another country's vessels on the high seas. As a result, at this stage, they are being held in custody while decisions are made on their future."

He continued, "The two boarded the Yushin Maru after they made attempts to entangle the screw of the vessel using ropes and throwing bottles of acid onto the decks."

Conservationists have been chasing the fleet in an attempt to stop them hunting near the Antarctica coast.

The fleet plans to kill about 900 minke whales and 50 fin whales by mid-April.

The hunt is part of what it calls a scientific research program, permitted under a clause in International Whaling Commission rules.

Campaigners say that is a front for commercial whaling.

The protesters have been divided on how best to stop the whalers.

Greenpeace has pledged to take non-violent action, but the Sea Shepherd has advocated direct intervention.