Armed Russian officers stormed Thursday a Greenpeace vessel that is in the Arctic to protest oil exploration by Russian gas giant Gazprom, the environmental group said.

"People armed with machine guns have descended on to the Greenpeace icebreaker Arctic Sunrise from a helicopter via ropes," Greenpeace said in a statement.

On the Twitter page of the Dutch-flagged Arctic Sunrise, activists said that the men, apparently the Russian coastguard are "onboard with guns. They are breaking into the comms rooms now."

Most of the crew of the Arctic Sunrise ship has been ordered at gunpoint to the upper deck, Greenpeace added on the Russian Twitter blog.

Greenpeace says the authorities are holding two activists, from Switzerland and from Finland, on the ship of the Coast Guard for the second day without an explanation, after arresting them for attempting to scale the oil platform Prirazlomnaya.