MOSCOW – Russia opened a criminal case Tuesday against Greenpeace activists, accusing them of piracy for attempting to stage a protest on an Arctic oil rig. A Greenpeace spokeswoman called the accusation “absurd.”

Russian border troops seized the Greenpeace ice-breaker Arctic Sunrise, along with its multinational crew of 30 activists and sailors, in a dramatic commando operation in the Barents Sea on Thursday. The day before, the group had been foiled while attempting to raise a protest banner on a Russian oil drilling platform.

The ship was towed by the Russian coast guard to an anchor in Kola Bay, about six miles from the port of Murmansk.

“After conducting a preliminary investigation, the Russian Investigative Committee’s northwestern branch initiated a criminal case on the signs of … piracy committed by an organized group,” Vladimir Markin, the investigative committee spokesman, said in a statement published on the agency’s website Tuesday.

No formal charges have been filed. Piracy carries a potential sentence of five to 15 years in prison.

Greenpeace has said the group intended to raise a banner on the Prirazlomnaya drilling platform to protest Arctic pollution. Prirazlomnaya is a major Arctic oil exploration project of Gazpromneft, a subsidiary of the Russian state-owned energy giant Gazprom. It lies within a Russian exclusive economic zone.

Greenpeace spokeswoman Maria Favorskaya said Russian authorities had acted illegally in seizing and towing the group’s ship, which was traveling under the Netherlands flag.