Russia has blocked a Greenpeace ship from entering Arctic waters where the environmentalist group was planning to protest against oil exploration activities by Rosneft and Exxon Mobil, the group has said.

Russian authorities had denied the icebreaker Arctic Sunrise entry to the Northern Sea Route, citing questions over the vessel's ice strengthening, Greenpeace said in a statement.

It said the Arctic Sunrise has a higher ice classification than many of the more than 400 vessels that have been granted access to the northern sea route this year.

"This is a thinly veiled attempt to stifle peaceful protest and keep international attention away from Arctic oil exploration in Russia," Greenpeace campaigner Christy Ferguson said.

"The Arctic Sunrise is a fully equipped icebreaker with significant experience of operating in these conditions, while the oil companies operating here are taking unprecedented risks in an area teeming with polar bears, whales, and other Arctic wildlife."

Russia's northern sea route administration referred calls seeking comment to its transport ministry, which did not respond.

Greenpeace said it wanted to expose the offshore activities of Russian oil company Rosneft and its US partner Exxon Mobil in the Kara Sea, north of western Siberia. The companies are preparing to begin drilling operations there next year.

Greenpeace and other environmentalists have warned that drilling in the remote and icy Arctic could lead to devastating spills, threatening fish and wildlife already under pressure from climate change.

The activists have scaled offshore platforms in waters off Greenland and northern Russia, stunts that were carried out to draw attention to the oil industry's move into the Arctic.

US officials estimate the region holds up to 13% of the world's undiscovered oil and 30% of its untapped natural gas. Climate change is expected to make those deposits easier to reach as the Arctic ice cap shrinks.

The melt is also opening up Arctic sea lanes like the Northern Sea Route, where shipping activities are growing rapidly.