The government may press charges under domestic law against a New Zealand activist who secretly boarded a Japanese whaling ship in the Antarctic Ocean, Chief Cabinet Secretary Hirofumi Hirano said Tuesday.

Activist Pete Bethune of the group Sea Shepherd Conservation Society is being held aboard the Shonan Maru No. 2 after he boarded the vessel Monday ostensibly to "arrest" its skipper for sinking his boat, the Ady Gil, in a collision last month.

"There are allegations that (Bethune) may have breached the criminal law by intruding into a vessel of our country," Hirano said at a news conference, noting the government is weighing various options, including turning the activist over to the Japan Coast Guard for questioning in Japan.