Russia has pressed piracy charges against all 30 people arrested after the environmental group Greenpeace staged a protest at an offshore oil platform in the Arctic.

The accused could be sentenced to 15 years in prison if convicted over the protest last month.

During the protest, a Greenpeace ship approached a platform belonging to state-controlled energy firm Gazprom and two activists tried to scale the rig.

Greenpeace said the protest was peaceful and has dismissed the piracy charges as absurd and unfounded.

"Our activists have been charged with a crime that did not happen, they are accused of an imaginary offence," said Greenpeace International Executive Director Kumi Naidoo.

The 30, who were aboard the Greenpeace ship Arctic Sunrise during the protest, are being held in custody in the northern Russian city of Murmansk.

They include activists and crew members from 18 nations as well as a British videograper and a Russian photographer.

Russia's Investigative Committee, which answers to President Vladimir Putin, said all the accused had denied guilt and refused to give substantive testimony. It said the investigation was continuing.

The Prirazlomnaya platform in the Pechora Sea is Russia's first offshore Arctic oil rig.