The activists, from the US environmental group Sea Shepherd Conservation Society, were held on the Japanese harpoon boat for two days after they delivered a letter protesting the slaughter of whales.

Only an hour after the two men were handed to an Australian customs boat on Friday, the crew of a Sea Shepherd ship hurled butyric acid bombs, or "stink bombs," onto the deck of the harpoon boat.

Japanese Foreign Minister Masahiko Komura told Australian Trade Minister Simon Crean in Tokyo that the actions of the group posed a danger and he urged cooperation to prevent "the recurrence of such an incident," according to the ministry statement.

He also asked Canberra to "take appropriate action" under national laws "should the Sea Shepherd boat call at an Australian port."

Crean, during talks here on bilateral ties, regional cooperation and the whaling issue, said the Australian Federal Police are investigating the case and that his government would decide on a response based on the results.

But as he did so, Australia moved to film Japanese ships in a bid to launch a legal challenge against the hunt's activities.

Australia has long opposed the hunt in the area, which includes a self-declared sanctuary, but has stopped short of physically intervening.

A customs ship tracked down the whalers and sent officers in smaller boats to gather video and photographic evidence, said a spokeswoman for Home Affairs Minister Bob Rebus.

"They are trying to get closer to see what the whaling fleet is doing," she said on condition of anonymity.

The customs ship, Oceanic Viking, had lost contact with the whalers after picking up the two activists after last week's stand-off.

Australia's Labor government vowed when elected last year to collect evidence of Japanese whaling in Antarctic waters, stepping up pressure on Tokyo to end its annual hunt.

On Tuesday, meanwhile, an inflatable boat from another group, Greenpeace, temporarily blocked a Japanese whaling ship from being refuelled by a Panamanian-flagged oil ship in Antarctic waters, the Japanese Fisheries Agency said.

"Despite warnings from the Nisshin Maru, Greenpeace went ahead with the interference, resulting in a moment of danger as the rubber boat temporarily tangled with wires of the refuelling ship," the agency said in a statement.

Ships from the environmental groups have been in the icy waters off Antarctica to prevent six Japanese whalers from carrying out their annual whale hunt which this year will see about 1,000 of the giant creatures slaughtered.

Japan exploits a loophole in a 1986 international moratorium on commercial whaling to kill the animals for what it calls scientific research, while admitting the meat from the hunt ends up on dinner plates.

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