Greenpeace protesters board Statoil's Arctic drilling rig

OSLO (Reuters) - Activists from environmental group Greenpeace have climbed aboard an oil drilling rig in the Norwegian Arctic on Tuesday, trying to stop Statoil's exploration plans in one of the world's northernmost prospects, the group said on Tuesday.

Greenpeace, which regularly calls Norwegian state-owned Statoil an 'Arctic aggressor', said plans to drill in the Hoop area of the Barents Sea threaten Bear Island, an uninhabited wildlife sanctuary which is home to rare species and occasionally to polar bears.

Oil firms are drilling further north in Norway than ever before as the Arctic ice retreats and recent regulation changes let firms work in areas where winter ice was common just decades ago.

Greenpeace climbed the rig, owned by Transocean, in the early morning hours, hung banners like 'No Arctic Oil' and 'Stop Statoil's Arctic Race', and said they were prepared to stay on the rig for days.

The move comes just weeks after Greenpeace tried unsuccessfully in Rotterdam to block the delivery of Russia's first oil from its Prirazlomanaya oil platform in the Arctic Pechora Sea.

The platform was briefly occupied by Greenpeace activists last year before they were arrested by Russian military forces and charged with piracy, carrying a potential prison term of decades, but released under an amnesty initiated by President Vladimir Putin.

Statoil called the latest move illegal and irresponsible and rejected Greenpeace's claims about safety.

"Hoop is an area with known geology, low pressure and temperature, and where Statoil has robust plans for the operations.

"An oil spill is very unlikely, at the same time we have put in place a number of barriers to be able to handle a situation should it occur," the company said.

The rig is currently in transit to the Hoop area where it has permission to start drilling in the Apollo prospect. However, it cannot drill into oil-bearing rock because a complaint from Greenpeace is pending.

(Reporting by Balazs Koranyi; Editing by Greg Mahlich)