Four Greenpeace activists climbed onto an Arctic oil rig Tuesday and halted drilling in a bid to pressure Britain's Cairn Energy to stop operations off Greenland's coast, the group said.

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

Greenpeace said its four activists, all expert climbers, had at dawn "dodged Danish Navy commandos before climbing up the inside of the rig and hanging from it in tents suspended from ropes, halting its drilling operation."

The climbers had supplies to allow them to remain in the tents for days, the group said.

By stopping drilling for just a short time, the activists could hinder Cairn Energy from completing exploration before winter and force it to postpone its search for oil off Greenland until next year, it 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," said one of climbers, Sim McKenna of the United States.

"The BP Gulf oil disaster showed us it's time to go beyond oil. 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," he said in the statement.

Cairn Energy, which is drilling two wells off the west coast of Greenland, said last week it had discovered gas and oil during its first exploration of the area.

The company countered environmentalists' concerns by saying its exploration complied with "some of the strictest regulations in the world" that are laid down by the government in Greenland, a semi-autonomous Danish territory.