Tokyo (CNN) -- Three Australian activists are being held aboard a Japanese ship Monday after illegally boarding the vessel to protest Japan's annual whale hunt in Antarctic waters.
The three men are not yet under arrest, but are being questioned by Japan's Coast Guard, a spokesman for Japan's Fisheries Agency said Monday, declining to be identified as is customary in Japan.
The Australian attorney-general, Nicola Roxon, said that her government was working to secure the release of the men, but that she believed they could face criminal charges in Japan.
Video footage released by the anti-whaling group Sea Shepherd shows the three men, from the environmental group Forest Rescue Australia, approaching the Japanese vessel on a small boat on Sunday.
The video shows their boat pull up next to Japan's Shonan Maru #2, a patrol vessel supporting Japan's whaling mission in the Antarctic waters. A voice from the small boat calls out, "Go, go, go!" as the men slip onto the Japanese ship. In the background, voices in Japanese scream in alarm, saying "They're boarding!"
Sea Shepherd described the three activists -- Geoffrey Owen Tuxworth, 47, Simon Peterffy, 44, and Glen Pendlebury, 27 -- as "prisoners." The organization said the men boarded the vessel to force Japan's fleet to stop hunting whales.
Japan annually hunts whales despite a worldwide moratorium, utilizing a loophole in the law that allows for killing the mammals for scientific research.
Each year, environmental groups like Sea Shepherd face off with Japan's hunters in a high seas drama that has led to collisions of ships, the detaining of activists and smoke bombs fired back and forth between the groups.
Two days before he boarded the Japanese vessel, Peterffy said in a video released by Sea Shepherd that stopping Japan's hunts was a personal responsibility.
"We've got a big responsibility for all those people in future generations," he said.
Australia's Roxon said the countries were engaged in diplomatic discussions to ensure the well-being and safety of the three men, but called it a "difficult situation."
Roxon said that while Australia steadfastly opposes Japan's whale hunt, the incident did not happen in Australian territorial waters.
"That doesn't give us rights for Australian law to automatically apply," she said. "In fact, our advice is that Japanese law will apply because a Japanese boat is the one that's been boarded."
This is the third time activists have boarded ships involved in the Japanese whaling fleet.
In 2008, two Sea Shepherd activists sneaked aboard a Japanese vessel. They were released back to the anti-whaling group.
In 2010, Pete Bethune, a New Zealander, also boarded the Shonan Maru #2.
Bethune was arrested, taken back to Japan, and tried in a Tokyo court. He spent five months in prison and was released on a two-year suspended sentence.