A Japanese whaling ship sent up a distress signal in the Antarctic after it reportedly collided with a boat carrying anti-whaling activists.
Activists have pursued the whaling ships for weeks
Campaigners for the US-based Sea Shepherd group said the clash happened as they tried to block the Japanese ship's access to a pod of whales.
They said their boat suffered a 1m (3ft) gash to its hull, and the whaling ship's propeller had been damaged.
Japanese officials said the group was behaving like pirates.
Anti-whaling activists have been pursuing the Japanese whaling ships for weeks, in the icy southern waters, as they hunt 850 minke whales and 10 fin whales.
A temporary truce was called on Friday when the whalers joined a hunt for two activists, who became lost during a confrontation.
The Sea Shepherd Conservation Society said the latest collision happened when its two boats moved in to stop the Japanese whaling ship Kaiko Maru, as it tracked some whales.
"They backed into us," the group's spokesman, Paul Watson said, adding that their ship the Robert Hunter had received a "gash in the steel of the hull".
He told AFP news agency that the Kaiko Maru's crew "are saying they have got a damaged propeller".
New Zealand's Rescue Co-ordination Centre confirmed it was aware that a Japanese ship had sent out a distress signal and that it was investigating.
No-one was believed to have been hurt, and no serious damage to either ship was reported.
The incident was condemned by the Japanese authorities.
"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," Hideki Moronuki, of Japan's fisheries agency, said.
Mr Watson admitted the situation was dangerous. "It's a circus, that's for sure. But it could all be prevented by upholding international law," he said.
The International Whaling Commission (IWC) imposed a moratorium on commercial whaling in 1986.
Japan - which says its hunt is for scientific research purposes - hosts a meeting of IWC members this week, to push for a partial resumption of commercial hunts.