The government on Monday slammed the conservation group Sea Shepherd Conservation Society for employing dangerous tactics to obstruct the operations of Japanese whaling vessels in the Antarctic Ocean over the weekend, causing one of them to suffer damage to its stern.

“It was a very dangerous act and can never be condoned,” Chief Cabinet Secretary Yoshihide Suga told a press conference.

Japan has asked the Netherlands to prevent a recurrence, as the Sea Shepherd group’s vessels fly under Dutch flags, he said.

According to a statement released by the Japanese fleet engaged in so-called research whaling, the Japanese harpoon vessels Yushin Maru, Yushin Maru No. 2 and Yushin Maru No. 3 “were subject to sabotage” by the Sea Shepherd ships Steve Irwin and Bob Barker.

It said inflatable boats sent from the Sea Shepherd vessels deployed ropes in front of the bows of the Japanese vessels, one of which became entangled in the Yushin Maru’s propeller, while the Bob Barker collided with the stern of the Yushin Maru No. 3, bending its rail and denting the hull.

Sea Shepherd called Japan’s account of what happened “an absolute lie” and released footage it said shows an “unprovoked, ruthless and premeditated” attack by the Japanese vessels on the Sea Shepherd ships that lasted for nine hours Sunday morning.

It said the Japanese vessels crossed the bows of the Sea Shepherd ships dozens of times, towing 300 meter-long steel cables intended to ensnare and damage their propellers.