Eleven Nobel Peace Prize winners have called on Russian President Vladimir Putin to ensure that "excessive charges of piracy" laid against 30 Greenpeace activists are dropped, Greenpeace said on Thursday.

"We are writing to ask you to do all you can to ensure that the excessive charges of piracy against the 28 Greenpeace activists, freelance photographer and freelance videographer are dropped, and that any charges brought are consistent with international and Russian law," the Nobel laureates said.

"We are confident that you share our desire to respect the right to nonviolent protest," they said in a letter released by Greenpeace.

The Russian authorities have charged the 30 crew members with piracy, which carries a maximum sentence of 15 years, after they staged a protest against Arctic oil drilling last month.

The activists from 18 countries have been placed in pre-trial detention until late November, where their lawyer said they have to endure "inhuman conditions."

The Nobel laureates including South African Bishop Desmond Tutu and former President of East Timor Jose Ramos Horta said that an oil spill in the Arctic would have a "catastrophic impact" on local communities.

"We, like millions of people around the world, are watching this case, eager to see Russian authorities drop the piracy charges, treat the 'Arctic 30' in accordance with international law, reaffirm the right to nonviolent protest, and rededicate efforts to protect the Arctic."

Putin has said that the activists from Greenpeace's Dutch-flagged vessel "of course are not pirates" but his spokesman later said the president had expressed his own opinion.

The Netherlands has filed a legal case over the crew's arrest and on Wednesday, German Chancellor Angela Merkel voiced concern in a phone call to Putin over the detention of the Greenpeace activists.

On Thursday, a court in the northern city of Murmansk rejected bail requests from two more Greenpeace activists, Keith Russel from Australia and Mannes Ubel from the Netherlands.

The court earlier turned down bail pleas from other Greenpeace crew members including Arctic Sunrise captain Peter Willcox and freelance photographer Denis Sinyakov.

Critics have said even if two of the Greenpeace activists might have violated the law by trying to scale a state oil platform it was unfathomable why two freelance journalists, a cook and a doctor accompanying the ship were facing piracy charges.

Russian doctors were collecting signatures on a petition to call on the authorities to free doctor Yekaterina Zaspa, saying keeping her in detention was "a terrible cruelty".

Alexei Venediktov, the well-connected editor of Echo of Moscow radio, said this week he had discussed the Greenpeace arrests with a "Kremlin friend."

He said he had told his influential friend that the Kremlin officials were making "clowns" of themselves by letting investigators press piracy charges against the ship's auxiliary staff like the cook.

"He looks at me and says: 'Silver was also a cook,'" Venediktov quoted the Kremlin official as saying, referring to the pirate Long John Silver from "Treasure Island" a novel by Robert Louis Stevenson.