(CNN) -- Australian authorities said Thursday that a customs vessel would pick up two activists currently aboard a Japanese whaler in Antarctic waters, news reports said.

The reports came as an anti-whaling group accused the crew of a Japanese vessel of kidnapping two activists who climbed on board the ship to try to stop its whaling operations in the Southern Ocean.

The incident caused Japan to contact the Australian government to help arrange the return of the two activists, The Associated Press reported.

Australian citizen Benjamin Potts and British citizen Giles Lane -- both members of the Sea Shepherd Conservation Society -- came on board the Yushin Maru No. 2 on Tuesday.

They tried to deliver a letter saying the vessel was violating international law and Australian law by killing whales.

A video from Sea Shepherd shows the two men tied to the ship's railing at one point while Japanese fishermen pace back and forth in front of them.

"They were seized by the crew and assaulted," said Capt. Paul Watson, founder of Sea Shepherd.

"They're being held hostage, they've been kidnapped and the Japanese are trying to use them to try and extort an agreement out of us, which is to leave them alone ... which to me is a form of terrorism." Watch Watson describe the incident »

But the Japanese Fisheries Agency charged that the Sea Shepherd members were the terrorists.

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The agency released pictures of broken bottles it claims the activists threw at the ship. It also showed a photo of the two men relaxing and drinking tea aboard the Yushin Maru.

Still, the image of the two men tied to the ship's railing is the one that has caused concern, leading to a call from the Australian government for the men's immediate release.

"For some time, for 10, 15 minutes, I understand, they were tied to a GPS mast," Tomohiko Taniguchi of Japan's Foreign Ministry told CNN. "The Japanese crew members feared that two crew members from Sea Shepherd might do something violent."

Potts and Lane boarded the vessel without permission, he said.

Watson countered the two boarded only after attempts to contact the ship by radio were unsuccessful.

Japan's Foreign Ministry said it spent most of Wednesday trying to get the men released, but that Sea Shepherd was not answering its phone calls. Sea Shepherd, meanwhile, told CNN it has not received a call from the Japanese government.

The impasse has led Tokyo to contact Australian authorities for assistance.

"It has become apparent that it will be impossible to hand the two trespassers back directly to Sea Shepherd, so our only option at this point is to make contact with another ship such as the customs vessel Australia dispatched," said Hideki Moronuki, a spokesman for the Japanese Fisheries Agency's whaling section, said in an AP report.

Australian Foreign Minister Stephen Smith said in an AP report that Canberra officials were considering the customs ship Oceanic Viking as a means of transferring the two activists.

Japan has been hunting whales in the Antarctic and apparently plans to kill as many as 1,000 this winter. The killings are allowed under international law because their main purpose is scientific.

"We regard them as poachers," Watson said.

Sea Shepherd claims Japan's Institute for Cetacean Research -- which is backing the operations -- has said it will release the two activists if Sea Shepherd agrees to stop interfering in its whaling operations. The group says it will not agree to that demand.

The Sea Shepherd vessel, the Steve Irwin, was no longer in sight or radar range of the Yushin Maru No. 2 on Wednesday, the group said in a statement.

"The good news is they haven't killed any whales for a week," Watson told CNN. E-mail to a friend

CNN Correspondent Kyung Lah contributed to this report.

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