The 30 activists from 18 countries were on a Greenpeace ship, the Arctic Sunrise, which was seized last week by the Russian Coast Guard. The ship was towed Tuesday into a small bay near Russia's Arctic port of Murmansk.

The Investigative Committee, Russia's main federal investigative agency, said its agents will question all those who took part in the protest and detain the "most active" of them on piracy charges. Piracy carries a potential prison sentence of up to 15 years and a fine of 500,000 rubles (about $15,500).

Two activists tried to climb onto the Prirazlomnaya platform on Thursday and others assisted from small inflatable boats. The Greenpeace protest was aimed at calling attention to the environmental risks of drilling for oil in Arctic waters.

"When a foreign vessel full of electronic technical equipment of unknown purpose and a group of people calling themselves members of an environmental rights organization try nothing less than to take a drilling platform by storm, logical doubts arise about their intentions," Investigative Committee spokesman Vladimir Markin said in a statement.

He said the activists posed a danger to operations on the oil platform. "Such activities not only infringe on the sovereignty of a state, but might pose a threat to the environmental security of the whole region," Markin said.

The oil platform, the first offshore rig in the Arctic, was deployed to the vast Prirazlomnoye oil field in the Pechora Sea in 2011 but its launch has been delayed by technological challenges. Gazprom has said it was to start pumping oil this year, but no precise date has been set.

Greenpeace insisted that under international law Russia had no right to board its ship and has no grounds to charge its activists with piracy.

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