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

The charge against the activists -- a Briton, a Finn, a Brazilian, a Russian and a dual US-Swedish citizen -- dimmed hopes that 25 others detained over the protest could be indicted on a lesser charge.

Piracy by an organised group carries a punishment of between 10 and 15 years.

A spokeswoman for the Moscow-based Investigative Committee declined to provide details of the charges, confirming only that the activists from the group's Dutch-flagged Arctic Sunrise icebreaker detained for two months were being charged.

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.

"Any claim that these activists are pirates is as absurd as it is abominable. It is utterly irrational, it is designed to intimidate and silence us, but we will not be cowed."

He said the charges represented the "most serious threat" to environmental activism since the group's ship Rainbow Warrior was bombed and sunk by French special services in New Zealand in 1985.

The charges of piracy came despite President Vladimir Putin's statements 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.

Putin's spokesman Dmitry Peskov told AFP Wednesday that Putin had expressed his personal opinion.

"He is not an investigator, nor a prosecutor, judge or defence lawyer."

Among those already charged were Brazil's Ana Paula Alminhana Maciel, British freelance videographer Kieron Bryan, and Finnish activist Sini Saarela, who was one of the climbers who attempted to scale a Russian oil platform.

The other two charged were spokesman Dmitri Litvinov, a dual US and Swedish citizen, and Russia's Roman Dolgov.

Greenpeace says it will appeal and turn to the European Court of Human Rights.

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 accused 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 2,000 kilometres (1,250 miles) north of Moscow and above the Arctic Circle.

All but four of them are non-Russians from countries including Britain, the United States, Finland and Argentina.

The Arctic Sunrise crew members are "close to shock" over their conditions in their jails, an activist said.

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

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

The arrests have also sparked outrage from rights activists.

"It is a Kafkaesque story," Tanya Lokshina, deputy head of Human Rights Watch in Russia, told AFP. "We could not have imagined this is possible."

Greenpeace held a similar protest at the same oil platform last year without incurring any punishment.

The unusually tough charges are likely to generate new controversy as Russia gears up to host the Winter Olympics in February already overshadowed by claims of gay rights abuse.

Since returning to the Kremlin for a third term as president last year, Putin has sought to re-assert his control over the country after huge protests in winter 2011-12.

Last month, Putin lambasted the West and pledged to fiercely protect Russia from foreign influence, saying its sovereignty and independence were "red lines" that could not be crossed.

On Saturday, a pro-Kremlin NTV channel broadcast a smear documentary claiming Greenpeace served US corporate interests and made huge profits staging protests against Russian rather than US industries.

The group has denied the accusations.