Thirty activists of Greenpeace who attempted to climb onto an Arctic offshore drilling platform last week have been interrogated for several hours before being taken to a detention facility.

Russia's main federal investigative agency said it would prosecute the activists on piracy charges for trying to access the platform owned by the state-controlled gas company Gazprom.

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

The Greenpeace protest was aimed at calling attention to the environmental risks of drilling for oil in Arctic waters.

The Investigative Committee said its agents would 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 ($15,500).

'Not pirates"

Meanwhile, the Russian President Vladimir Putin said though the activists were not pirates, they had broken international law.

"It is absolutely evident that they are, of course, not pirates, but formally they were trying to seize this platform. It is evident that those people violated international law," Putin said at an Arctic forum on Wednesday.

The oil platform, the first offshore rig in the Arctic, was deployed to the vast Prirazlomnoye oilfield 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.