Russia is moving the imprisoned crew of Greenpeace's Arctic Sunrise protest ship from their jail in the Arctic Circle city of Murmansk to Saint Petersburg, the organisation said Friday, quoting information from diplomats.

The 30 crew members, including 26 foreigners from 18 countries, who were arrested in September after protesting oil exploration in the Barents Sea "are being moved from a detention centre in Murmansk to a jail in Saint Petersburg," Greenpeace said in a statement.

"We had various diplomats confirm this information," spokeswoman Dannielle Taaffe told AFP. She said Greenpeace does not yet understand the reasons for the move.

Russia's prison service denied late Friday that the activists were already in Saint Petersburg, news agencies reported.

The crew, comprised of 28 activists and two journalists, have been placed in pre-trial detention until November 24. Several members have complained of cold cells and a lack of suitable clothing and food.

They were detained and charged with piracy after their ship was boarded by Russian coastguards.

Last Wednesday, the Investigative Committee said it had reclassified the crime as hooliganism, a lesser charge, but Greenpeace on Friday said that the activists never received official documents formally lifting the piracy charge.

"Essentially, right now all 30 people have two charges," spokeswoman Maria Favorskaya told AFP. While piracy carries a punishment of up to 15 years in prison, hooliganism can be punished by a maximum of seven.

Investigators in Murmansk previously told AFP that the entire high-profile probe was being handled from Saint Petersburg, rather than by the local Investigative Committee.

One of Russia's northernmost cities, Murmansk endures polar nights in the winter, with temperatures often dropping to below minus 30 degrees Celsius (minus 22 degrees Fahrenheit).

Several activists in mid-September attempted to scale Russia's Gazprom oil platform in the Pechora Sea, part of the Barents Sea, in protest of its exploration in the Arctic.

The Greenpeace ship came under control of Russian authorities after it was boarded on September 19. It was towed to the Murmansk port. Greenpeace says the authorities had no right to detain the Dutch-flagged ship in international waters.

French Prime Minister Jean-Marc Ayrault, who on Friday met in Moscow with his counterpart Dmitry Medvedev and President Vladimir Putin, asked the Russian leaders to release the French national among the crew.

"I asked that he would make a humanitarian gesture toward (French activist Francesco Pisanu)," Ayrault told journalists after meeting with Medvedev.

But Medvedev responded that Russia will not condone attempts to "influence technically complicated and unsafe processes" associated with the extraction of natural resources, because objects such as oil platforms are essentially "like a powder keg".

The Netherlands has taken legal action against Russia, with a first hearing scheduled for next Wednesday at the International Tribunal for the Law of the Sea in Hamburg, Germany.

Russian authorities initially accused the activists of carrying out illegal research activities, then charged them with piracy.

Last month the Investigative Committee also said that the ship carried illegal drugs such as poppy straw and morphine, which Greenpeace denied.

The platform is located in Russia's exclusive economic zone on the Arctic shelf, which means that most Russian laws do not apply there.