Greenpeace activists on Friday raided an Arctic oil rig owned by Russian group Gazprom that next year will controversially pioneer commercial drilling in one of the world's last pristine reserves.

Russia's largest energy group doused the six-member team from helicopters with icy streams of water as they hung down on thick blue ropes from the side of the huge Prirazlomnaya platform.

Angry workers meanwhile pelted them with chunks of metal.

"Not just hosed water, but now metal being thrown by #Gazprom crew at our activists; we're coming down," Greenpeace International's Executive Director Kumi Naidoo tweeted as the 15-hour sea saga wound down.

"The bravery of all of these climbers interrupted a major Arctic oil operation, and by doing so brought the world's attention to this era defining issue," the group added in a statement issued after the team climbed off.

It was not immediately clear whether anyone had been detained or faced Russian charges.

The daring raid came as Russia takes the lead from other Arctic energy powers in exploiting previously untouched territory for what is believed to be one of the world's largest holdings of recoverable oil and natural gas.

Gazprom's independent project is due to kick off next year just as fellow state oil firm Rosneft begins its own initial explorations with new partner ExxonMobil.

The area -- also the subject of territorial rows with resource rivals Canada and Norway -- is becoming especially attractive as the size of the ice shelf shrinks and conflicts continue to rattle energy producers in the Middle East.

The dramatic adventure began with a carefully planned pre-dawn sneak attack in which the activists raced up to the floating production base on inflatable speedboats that came off Amnesty's Arctic Sunrise icebreaker.

Footage shot by one of the crew showed the sea calm but draped in metallic clouds as the tiny bright orange craft sped through unguarded waters toward the towering crane-mounted station.

The team then threw up mooring lines and climbed on the side of the rig before unfurling neon yellow banners reading "Save the Arctic!" and "Stop Gazprom!"

The state-owned firm immediately denied any impact on operations and said the activists had in fact turned down a cordial offer to enter the base for talks.

"They were invited aboard the platform for a constructive dialogue," a Gazprom spokesman told Russian news agencies.

"But they refused and said they would prefer to hang off the platform instead."

Gazprom next year will become the first company to start commercial drilling in the Arctic when it launches offshore operations in the southeastern section of the Barents Sea.

The holding's base runs just west of projects being pursued jointly by ExxonMobil and Rosneft in an area viewed by the Kremlin as the main source of Russia's future oil and gas wealth.

But critics warn that Gazprom's drilling is extremely risky because the platform is sealed in ice for most of the year and has to work smoothly in temperatures that often plunge to minus 50 degrees Celsius (minus 58 Fahrenheit).

The Gazprom unit plans to drill and process oil before injecting it into tankers -- operations that have never been performed in such an inhospitable climate before.

Critics say the risk of such work far outweighs the benefits it may offer either the Russian government or consumers through cheaper fuel.

"The Prirazlomnaya platform will produce no more than seven million tonnes of oil a year," Greenpeace Russia director Vladimir Chuprov told Moscow Echo radio.

"And the country needs to produce 500 million tonnes a year."