Russia on Tuesday put 30 crew members of a Greenpeace protest ship in prisons in Saint Petersburg, after moving them from the Arctic Circle city of Murmansk where they have spent weeks in jail amid growing international concern.

The move to possibly milder and more comfortable conditions in Russia's second city came amid an apparent intensification of global pressure on Russia over the detention of the crew from 19 different countries, who have spent over six weeks in Murmansk.

German Chancellor Angela Merkel has already voiced concern over the case while British Prime Minister David Cameron last week urged President Vladimir Putin to treat the so called "Arctic 30" fairly.

The Russian prison service transported the prisoners in a special carriage attached to a regular passenger train, which arrived in a train station in Saint Petersburg on Tuesday afternoon, an AFP photographer saw.

A column of prison service trucks then drove away from the station.

It was not clear if the prisoners had been moved to ensure better detention conditions in the high-profile case or to ease the investigation which is being run from Saint Petersburg, Russia's northern capital.

Greenpeace wrote on Twitter that several male detainees had been driven to the notorious Kresty prison on the banks of the Neva River which housed political prisoners both in tsarist and Soviet times.

The striking red-brick complex built in 1890 counts Leon Trotsky among its former inmates. It is known by the nickname Kresty, or Crosses, because of its layout, although its official name is Investigative Isolator Number One.

According to Greenpeace, six of the 30 have been sent to Kresty. Three more men are in Saint Petersburg's detention centre Number Four while four women have gone to detention centre Number Five. The organisation was still confirming where the other activists are being held.

'Make sure they go home'

On arrival, the prisoners face an initial quarantine period when they are not allowed to meet lawyers.

Many countries have consulates in Saint Petersburg, Russia's second largest city, which could make it easier for diplomats to visit the prisoners. The city also has direct flights abroad.

The train journey from Murmansk to Saint Petersburg covers around 1,500 kilometres (950 miles) and takes 27 hours.

The crew members of Greenpeace's Arctic Sunrise ship were detained after several staged a protest against energy prospecting in the Arctic by scaling a state-owned oil platform on September 18.

Russian authorities boarded the ship a day later and towed it to Murmansk where it is still impounded. Greenpeace says the authorities had no right to detain the Dutch-flagged icebreaker in international waters.

The Netherlands this month took Russia to court, arguing in the International Tribunal for the Law of the Sea that authorities had no right to prosecute the crew or impound the ship. Russia has boycotted the hearings, but the tribunal will rule on the case on November 22.

The situation has already strained relations between Moscow and the Netherlands while Britain's Cameron described the charges as "excessive" and said he had raised the issue in a phone call last week with Putin.

"I have appealed to Vladimir Putin to try to de-escalate this and make sure that these people can go home," he told BBC Radio last week. Six of those detained are British.

Russia's Investigative Committee in October said it was changing the initial piracy charges against the crew members to hooliganism, an offence that carries a maximum penalty of seven years in prison.

But Greenpeace says the piracy charges were never formally lifted, meaning the activists are currently facing charges of both piracy and hooliganism.

All have been detained until November 24 and are likely to be convoyed to court hearings next week to extend their detention.

On Tuesday the Kremlin's human rights council published an open letter to the head of the Investigative Committee, asking for the Greenpeace crew members to be released on bail.

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