MOSCOW — Greenpeace said Wednesday that 14 of its activists who were detained after protesting at a Russian oil platform have been charged with piracy, a crime punishable by up to 15 years in prison.

The environmental activists from Argentina, Brazil, Britain, Finland, the Netherlands, Poland, Russia and Sweden were among 30 people from 18 countries who were on board the Greenpeace ship that was seized by the Russian coast guard after the Sept. 18 protest. Those charged Wednesday by the court in the Arctic city of Murmansk included 13 Greenpeace activists and a freelance British video journalist.

Two of the activists had tried to scale an offshore platform owned by Russian state-controlled energy giant Gazprom to call attention to the environmental risks of drilling in Arctic waters.

More activists were expected to be formally charged Thursday and Friday, Greenpeace said. All 30 were being held in jails in Murmansk, a port above the Arctic Circle.

The ship’s captain, Peter Willcox, is married to Maggy Willcox, publisher of the Islesboro Island News in Maine.

President Vladimir Putin said last week that he doesn’t think the Greenpeace activists are pirates, which triggered hopes for their release. But Prime Minister Dmitry Medvedev on Wednesday urged energy companies to adopt more stringent security measures, and said the government should consider tougher penalties for those who attack or trespass on the grounds of Russian oil and gas infrastructure.

“Concern for the environment must not cover up unlawful actions, whatever lofty goals the people who were taking part in them espoused,” Medvedev was quoted by Russian news agencies as saying.

The platform, which belongs to Gazprom’s oil subsidiary, is the first offshore rig in the Arctic. It was deployed to the vast Prirazlomnoye oil field in the Pechora Sea in 2011, but its launch has been delayed by technological challenges. Gazprom said in September that it would start pumping oil this year, but no date has been set.

Investigators said the Greenpeace ship, the Arctic Sunrise, had violated the 500-yard security zone around the platform and that it was carrying equipment with an unclear purpose. Greenpeace has insisted that its ship stayed out of this zone and that the inflatable boats used by activists to reach the platform posed no danger.

The activists charged Wednesday were called separately into the offices of the Investigative Committee, Russia’s main investigative agency, and presented with the formal charges in the presence of their lawyers, Greenpeace said. The Investigative Committee did not immediately announce the charges.