TOKYO - Japan plans to hold and may charge a New Zealand anti-whaling activist being held aboard one of its harpoon ships in Antarctic waters, officials said Tuesday.

Peter Bethune secretly climbed aboard the Japanese whaling fleet's security ship the Shonan Maru 2 planning to make a citizens' arrest of its captain Hiroyuki Komiya for "attempted murder".

He left the Sea Shepherd anti-whaling vessel on a jet ski before dawn on Monday, also planning to hand Komiya a US$3 million (S$4.2 million) bill for the destruction of the Ady Gil powerboat, of which he was captain.

The carbon-and-kevlar trimaran, which claimed the round-the-world record under its former name "Earthrace" in 2008, was destroyed when it collided with the whaling vessel on January 6. Six crew, including Bethune, were on board.

But Japan may now charge Bethune, who remains in Japanese custody on the ship, in the latest twist in the increasingly heated confrontations between the whalers and the militant Sea Shepherd activists.

Japan plans to question Bethune and establish a criminal case under Japanese law, the Kyodo news agency reported, citing unnamed sources.

Foreign Minister Katsuya Okada said Bethune was being kept aboard the Japanese ship under international maritime law.

"We are preparing to take him to Japan without releasing him to a third country," he said.

Fisheries Minister Hirotaka Akamatsu earlier signalled tough action against Bethune, telling reporters: "As it is outrageously illegal behaviour, we want to deal with it strictly."

Over recent weeks, the whalers and activists have blasted each other with powerful icy jets from high-pressure water cannon, while those aboard the Sea Shepherd have hurled rancid butter stink bombs at the whalers.

The anti-whalers have also accused the Japanese of using sonar riot-control devices against them, while the whalers say they have been hit with paint bombs and flashed with a high-powered laser-like weapon.

In New Zealand, Foreign Minister Murray McClully on Tuesday met with Japan's ambassador, the foreign ministry in Tokyo said.

Commercial whaling has been banned worldwide since 1986, but Japan justifies its annual hunts as "lethal scientific research", while not hiding the fact that the meat is later sold in shops and restaurants.

In a separate case, two Greenpeace activists went on trial in Japan this week for theft and trespass for taking a box of whale meat from a delivery service depot in 2008. They claim the meat was evidence of embezzlement in the state-funded whaling programme.

Bethune's case is not the first time Sea Shepherd activists have boarded Japanese whaling ships. In January 2008 a Briton and an Australian climbed aboard a Japanese harpoon vessel to deliver a protest letter.

After two days the Japanese side handed them back to an Australian customs boat.

The Sea Shepherd's Captain Paul Watson, in an online posting, said: "Captain Bethune was entirely in his rights to confront the man who almost killed him and destroyed his ship.

"And now this same Japanese captain who destroyed a ship almost killing its crew is intent on bringing Captain Bethune back to Japan as his captive. The question must be asked -- who are the pirates here?"