Russian investigators have charged five Greenpeace campaigners from several countries with piracy over an open-sea protest against Arctic oil drilling, the environmental group said, calling the move an "outrage".

The charge filed on Wednesday dimmed hopes that 25 others detained over would be indicted on a lesser charge.

Piracy by an organised group carries a punishment of between ten to fifteen years.

The accusation came despite President Vladimir Putin's statement last week that the activists "of course are not pirates." He, however, said that they did break the law by protesting dangerously close to an oil rig.

Greenpeace called the charges "extreme and disproportionate."

"This is an outrage and represents nothing less than an assault on the very principle of peaceful protest," Greenpeace International executive director Kumi Naidoo said.

Those already charged on Wednesday were Brazil's Ana Paula Alminhana Maciel, a crew member from Greenpeace's Arctic Sunrise icebreaker, freelance videographer Kieron Bryan from the UK, Finnish activist Sini Saarela, who was one of the climbers who attempted to scale an oil rig, Russian activist Roman Dolgov, and Dima Litvinov, an activist with Swedish and US citizenship.

Russian investigators accused the activists of piracy after several of them tried to scale state energy giant Gazprom's Prirazlomnaya oil platform in the northern Barents Sea last month.

The group has denied the charges and accuses Russia of illegally boarding its ship in international waters. The 30 activists from 18 countries are being held in pre-trial detention centres in the cities of Murmansk and Apatity, which are nearly 2,000km north of Moscow and above the Arctic Circle. All but four of them are non-Russians.

"Close to shock"

The Arctic Sunrise crew members detained in Russian jails for two months over their protest are "close to shock" over their conditions, a rights activist said Tuesday.

They have complained of cold cells and a lack of suitable clothing and food, Irina Paikacheva, the head of a state-connected regional prisoners' rights watchdog, told AFP.

"They had never expected that they would face such consequences for their peaceful protest in a democratic state."

The foreign detainees are struggling to make themselves understood since virtually none of the prison staff speaks English, she added.

The Dutch government has called on Moscow to release the activists immediately and said it was considering legal action.

The arrests have also sparked outrage from Russian and international rights activists, with Reporters Without Borders saying investigators were "criminalising both journalists and environmental activists".

Source: Agencies