The Japanese government said Monday two Japanese ships involved in so-called research whaling in the Antarctic Sea were hindered by the anti-whaling group Sea Shepherd Conservation Society.
According to the Fisheries Agency, two small boats deployed from a Sea Shepherd ship tried to interrupt the work of the Japanese vessels when crew members threw ropes to ensnare their propellers.
No crew members were injured in the confrontation and the damage sustained by the ships will not affect their operations, the agency said.
Japan asked the Netherlands, where the boats are registered, to "take effective measures" to prevent a recurrence, Chief Cabinet Secretary Yoshihide Suga told a press conference.
Sea Shepherd's acts were "extremely dangerous and we simply cannot accept them," the top government spokesman said.
Sea Shepherd, meanwhile, said in a statement the Japanese ships ambushed its vessel by trailing steel cables in an attempt to disable the conservation boat's propellers and rudder.
Japan's whaling activities, which it defends as being for research purposes, has drawn international criticism as being a cover for commercial whaling. The International Whaling Commission placed a zero-catch limit on commercial whaling in 1986.