BP is working around the clock to find the source of a leak that has shut down a 100,000 barrel per day gathering centre at Prudhoe Bay, Alaska, the biggest oil field in North America.

"It certainly is a significant spill," BP said. It said it was unclear how much oil had leaked out on to the tundra, though 625 barrels of oil and water had been recovered. Recovery efforts by workers with shovels and vacuums have been slowed by fears of slips on the icy tundra and of frostbite in freezing temperatures.