Protesters from environmental group Greenpeace boarded the Stena Don oil rig [Reuters/Greenpeace]

Four Greenpeace activists have scaled an oil rig off the coast of Greenland, forcing a British energy company to shut down its drilling operation in the Arctic.

The environmental campaigners managed to breach a 500 metre security perimeter around the Stena Don rig, climb the structure and fasten themselves to it, police said on Tuesday.

"Campaigners have evaded a huge military security operation to scale a controversial oil rig in the freezing seas off Greenland," Greenpeace said in a statement.

The action is the latest act by the environmental watchdog in its attempt to pressure Cairn Energy, a British oil exploration company, to stop drilling in the Arctic.

'Arctic oil rush'

In a statement, Sim McKenna, one of the climbers, said: "We've got to keep the energy companies out of the Arctic and kick our addiction to oil, that's why we're going to stop this rig from drilling for as long as we can.

"The drilling rig we're hanging off could spark an Arctic oil rush, one that would pose a huge threat to the climate and put this fragile environment at risk."

But Morten Nielsen, a police spokesman in Nuuk, Greenland's capital, said the activists would be arrested.

Ships towed and melted a large iceberg that floated near a Cairn Energy site off Greenland [Reuters/Greenpeace]

"When someone breaks the law - and it has happened here - the person or persons will be prosecuted," he told the AP news agency.

Police have been monitoring the activists from a Danish navy ship patrolling the area.

The navy said it had no immediate plans to remove the activists, explaining that it was up to police in Greenland, a semi-autonomous Danish territory, to decide what to do.

"Right now we're waiting to see what happens," Michael Hjort, a naval spokesman, told the Greenland newspaper Sermitsiaq.

"We are ready with our dinghies in case the activists fall in the water."

Last week, the Greenpeace ship Esperanza anchored near the rig, just as Cairn Energy announced it had discovered gas during its first exploration of the area.

The company is drilling two wells off the west coast of Greenland and is said to plan two more before the end of October.

Greenpeace is concerned that the find will spark an oil rush in an environmentally fragile area, home to blue whales, polar bears, seals and migratory birds.

Pictures emerged last week showing a ship hosing down and melting a giant iceberg that had drifted near the Stena Fourth drilling ship, also operated by Cairn Energy.

The company has said its exploration complies with "some of the strictest regulations in the world" that have been laid down by the government in Greenland.

Source: Agencies