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," they said in a letter.

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

On Wednesday, German Chancellor Angela Merkel voiced concern in a phone call to Putin over the detention of the Greenpeace activists, becoming the first non-Dutch politician to have publicly raised the issue with the Russian strongman.

Last week Russian investigators said "narcotic substances" had been found on the ship and several of the activists would face additional charges.