An anti-whaling group's boat and a Japanese whale-spotting vessel collided twice in Antarctic waters Monday during clashes over a pod of whales, conservationists and Japanese officials said.

The Canadian-founded anti-whaling group Sea Shepherd claimed a one-metre gash was torn in the hull of its ship, the Robert Hunter, by the Japanese ship Kaiko Maru during the clashes in iceberg-strewn waters far south of New Zealand.

Japanese officials accused the group of attacking the whaling ship like pirates.

The Kaiko Maru issued a distress signal during the clash to seek help from another Japanese whaling ship in the area, said Hideki Moronuki, a senior official at the far seas fisheries division of Japan's Fisheries Agency. No one aboard the Kaiko Maru was injured, and no serious damage to the ship was reported.

Moronuki claimed two Sea Shepherd vessels attacked the Kaiko Maru on Monday morning.

"The attack was like that of a pirate, with people on one boat throwing warning flares and a rope in an attempt to entangle our ship's propeller," Moronuki said, adding that the Kaiko Maru was forced to stop.

Five other ships in the Japanese whaling fleet were far away from the Kaiko Maru at the time of the collision, Moronuki said, declining to give their location.

Toronto-born Sea Shepherd founder Paul Watson claimed the Robert Hunter was hit twice by the Kaiko Maru after the conservationists tried to stop the Japanese ship from reaching a pod of whales.

"Robert Hunter was struck in the stern. We have a three-foot gash in the hull above the waterline," he said by telephone from the Farley Mowat, a second Sea Shepherd ship in the area.

No injuries were reported aboard the Robert Hunter.

Both Sea Shepherd ships are named after prominent Canadian environmental activists. 

Watson said the Sea Shepherd ships offered to respond to the Japanese ship's distress call, but it had not answered. The three ships were still near each other, while other Japanese whaling ships were about 30 kilometres away, Watson said.

"The situation clearly is dangerous," he said.

New Zealand's Rescue Co-ordination Centre said it was aware of a distress call from a Japanese vessel, and that authorities were investigating.

Clash not the first

The collision was the latest high-stakes clash between Japanese whaling ships and Sea Shepherd activists, whose self-stated aim is to "harass, block, obstruct, and intervene against" Japanese ships hunting whales in Antarctic waters.

Last Friday, two Sea Shepherd members went missing aboard a small inflatable boat for several hours during a confrontation with another Japanese whaling ship, the Nisshin Maru, before being found safe.

The conservationists had dumped a foul-smelling acid on the whaling ship, prompting Japanese officials to label them "terrorists" after two crew members were slightly injured.

The Japanese ships left port in November for a six-month whaling expedition in the Antarctic as part of a scientific whaling program, conducted within the rules of the International Whaling Commission.

Tokyo is pushing for a limited resumption of commercial hunts, arguing that whale stocks have sufficiently recovered since 1986, when a global moratorium on commercial whaling was introduced.

Many other countries and conservation groups say Japan's scientific program is a veil for commercial whaling.

With files from the Associated press