Of the thirty Greenpeace activists taken into custody over their peaceful protest against arctic drilling by oil giant Gazprom, five of them on Wednesday have been officially charged with piracy by Russian officials.

"A charge of piracy is being laid against men and women whose only crime is to be possessed of a conscience. This is an outrage and represents nothing less than an assault on the very principle of peaceful protest," said Greenpeace International executive director Kumi Naidoo.

The activists from the ship are from 18 countries, including Australia, Brazil, Canada, Denmark, France, New Zealand, Russia, the UK and the US.

Russian investigators said they had found drugs aboard a Greenpeace ship used in a protest against offshore Arctic drilling and would press new charges against some of the 30 people being held for alleged piracy.

Greenpeace Russia mocked the accusation. "The Investigative Committee 'found' narcotics. We are waiting for it to find an atomic bomb and a striped elephant," it said on Twitter. "This is possible in Russia these days and can hardly surprise anybody."