Russian investigators said Thursday they had charged all 30 crew members of Greenpeace's Arctic Sunrise ship with piracy over a protest against Arctic oil exploration, an offence that carries the risk of a lengthy prison term.

A court in the northern city of Murmansk last week detained the crew members including freelance journalists for two months pending an investigation into their protest on an oil platform owned by energy giant Gazprom.

"All 30 participants in the criminal case have been charged over the attack on the Prirazlomnaya platform," the Investigative Committee said in a statement.

"They are all charged with... piracy committed by an organised group."

Piracy by an organised group carries a prison sentence of up to 15 years in Russia.

Investigators accused the activists of trying to seize property with threats of violence.

The first 14 activists were charged on Wednesday and the rest indicted Thursday.

Greenpeace denies the crew members -- who come from 18 different countries including Britain, Russia, New Zealand, Canada and France -- committed any crime.

"Our activists have been charged with a crime that did not happen," Greenpeace International executive director Kumi Naidoo said in a statement.

"A profound injustice is right now being perpetrated against our friends, our brothers and sisters, our sons and daughters who sit in Russian jails."

British Foreign Secretary William Hague has met with Greenpeace executive director John Sauven to discuss "the arrest of six British nationals," his office said later Thursday.

Hague last week raised the issue with Russian foreign minister Sergei Lavrov, it said.

"We would remain in close contact with all other nations whose citizens were involved," Hague was quoted as saying.

Greenpeace spokesman Ben Ayliffe said one of the British activists had been taken ill prior to Thursday's hearing.

The September 18 protest saw several activists scale the oil platform in the Barents Sea to denounce Russia's plans to drill in the Arctic.

Russian border guards then lowered themselves onto the Dutch-flagged Arctic Sunrise from a helicopter, locked up the crew and towed the ship to Murmansk located nearly 2,000 kilometres north of Moscow.

Among those charged on Thursday was Russian freelance photojournalist Denis Sinyakov, a former AFP and Reuters staff photographer.

The Kremlin's council on human rights, an advisory body, said it was "extremely concerned" that the journalist covering the protest for a Russian online portal had been accused of piracy.

"We unambiguously consider the arrest and the laying of the piracy charges against Denis Sinyakov as pressure on the media," it said.

Leading Russian media last week blacked out photographs on their websites in protest at his detention.

Investigators on Wednesday charged a British freelance videographer.

Those charged Thursday included the ship's captain, American Peter Willcox.

He was the captain of Greenpeace's Rainbow Warrior ship, bombed by French agents in New Zealand in 1985.

The activists are being held in pre-trial detention centres in Murmansk and the nearby town of Apatity above the Arctic Circle.

Lawyers for the 30 have filed appeals against the decision to hold them in detention.

President Vladimir Putin has said that in his opinion the activists were not pirates but had breached international law by getting dangerously close to the oil rig.

'This is not justice, it's a reprisal'

Campaign groups including Human Rights Watch have called for their release.

The unusually tough charges for a protest has sparked comparisons with the case of the Pussy Riot punks who were last year sentenced to two years in a penal colony for demonstrating against Putin in a Moscow church.

"This all reminds me very much of the case of Pussy Riot," journalist Anton Orekh wrote on the website of popular radio station Moscow Echo.

"The whole world will rise up to defend them. And rightly so, because that's not justice but a reprisal."

Yet a poll carried out by state agency VTsIOM late last month found that 60 percent of respondents thought Russia's actions were appropriate.

A prisoners' rights activist told AFP this week the detainees were complaining of cold cells, chain-smoking fellow prisoners and difficulties communicating with guards, hardly any of whom speak English.

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