Russia's state-held energy giant Gazprom said Friday it had launched production at an Arctic oil rig raided in September by 30 Greenpeace activists whom the authorities later detained for two months.

The landmark announcement marked the formal start of Russia's long-planned effort to turn the vast oil and natural gas riches believed to be buried in the frozen waters into profits for its ambitious government-run firms.

But it also outraged campaigners who see the Arctic as one of the world's last pristine reserves whose damage by oil spills and other disasters would be enormously difficult to contain.

Gazprom made its announcement in a statement that stressed the company also had rights to 29 other fields it planned to exploit in Russia's section of the Arctic seabed.

"Gazprom has begun oil production at the Prirazlomnoye deposit," said Gazprom.

"This is the first project in Russia's history aimed at developing the resources of the Arctic shelf and the start of large-scale work by Gazprom that will create a major hydrocarbons production centre in the region."

The company -- already owner of the world's largest natural gas reserves and a growing presence in the oil sector -- said in planned to produce six million tonnes of crude per year (120,000 barrels per day) at the site by 2021.

It estimated Prirazlomnoye's oil reserves at 72 million tonnes -- a small field that would be responsible for just one percent of Russia's daily production and be depleted in about two decades.

But both Gazprom and the Kremlin view the field as a stepping stone in a much broader effort to turn the Arctic into the focus of future exploration that makes up for Russia's declining oil production at its Soviet-era Siberian fields.

Greenpeace responded to Gazprom's announcement by warning "that the clock is ticking on a major environmental accident in the Arctic region."

'Dark day for the Arctic'

Several Greenpeace activists aboard the Arctic Sunrise icebreaker attempted to scale the firm's Prirazlomnaya platform in a dramatic open-sea protest launched on September 18.

Russian coastguards and Gazprom crew fended the activists off by spraying them with freezing water and pushing them back into the heavy seas.

Armed security agents later boarded the Arctic Sunrise in a commando-style operations that resulted in the arrest of all 28 activists and their two accompanying journalists.

The group hailed from 18 countries besides Russia and saw governments ranging from Australia to Britain and the United States criticise the arrests.

The Greenpeace team was initially accused of piracy before those charges were reduced to the lesser crime of hooliganism.

Russia's parliament approved a Kremlin-backed amnesty this week that should soon end their prosecution and ensure their return home.

Arctic Sunrise member Faiza Oulahsen called Gazprom's past environmental safety record "appalling" and called the company's launch of production "a dark day for the Arctic."

"It is impossible to trust them to drill safely in one of the most fragile and beautiful regions on Earth," Oulahsen said in a statement.

But Gazprom said its Barents Sea rig "completely excludes the possibility of an oil spill during production and storage."

It added that the platform was equipped with a "zero-release system that excludes the possibility of drilling and production waste reaching the sea."

Control over energy fields in Russia's section of the Arctic is split between Gazprom and its state-owned rival Rosneft -- an oil producer that wants to break Gazprom's grip on the natural gas market.

Rosneft is partnering in the region with US major ExxonMobil and has smaller deals signed with Italy's ENI and Norway's Statoil.