Russian border guards stormed the ship from a helicopter late yesterday after Greenpeace activists scaled the platform, owned by state energy giant Gazprom, to protest plans to drill on the Arctic shelf.
Special forces armed with guns locked up the crew of about 30 people from Russia, the Netherlands, Finland, Switzerland and other countries and now control the ship, Greenpeace said.
Russia's powerful Investigative Committee said today border guards had seen evidence the crew may have committed piracy, which is carries a prison term of between five and 15 years in Russia.
Greenpeace is campaigning against surveying of oil and gas fields on the Arctic shelf, arguing that any oil leak would be catastrophic in the pristine environment and impossible to bring under control.
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The federal security service (FSB), which oversees the border guards, and the Russian foreign ministry have accused the activist ship of endangering lives, infiltrating the platform and conducting "extremist" activities.
Greenpeace expert Roman Dolgov told RIA Novosti from onboard the ship: "We are facing quite serious accusations: of terrorism and carrying out illegal scientific research."
A senior FSB official said there was evidence the crew were conducting "illegal scientific research."
"Upon inspection of Arctic Sunrise, we found large quantities of electronic equipment... And there is evidence of possible... Illegal scientific research," the head of FSB security services in Murmansk region, Mikhail Karpenko, told Rossiya24 television.