Between 760,000 and a million litres of crude leaked from a ruptured transit line onto the snow-covered tundra, according to an official estimate of the spill.

By comparison, the Exxon Valdez spilled 42 million litres when it ran aground in Prince William Sound in 1989, in the southern part of Alaska. 

Linda Giguere, of the state's Department of Environmental Conservation, said: "I can confirm it's the largest spill of crude oil on the North Slope that we have record of.

"It's a significant spill. The volume is large, but the footprint is small. It's contained and controlled, which is the really good news."

She said the state began comprehensive record keeping on spills 10 years ago, following years of cursory record keeping since the trans-Alaska oil pipeline was built in the 1970s.

Remote and unpopulated

The spill, discovered on 2 March, covers a hectare of remote and unpopulated land on Alaska's north coast near the Beaufort Sea.

Richard Fineberg, a former state oil analyst, said it is too early to determine environment consequences, but said the area, near the start of the trans-Alaska pipeline, does not match the popular image of the state.

"That area is not pristine. It's industrial," he said.

The source of the spill was a half-centimetre hole apparently caused by internal corrosion in the 5km line that leads to the trans-Alaska oil pipeline. It is not known when the leak started.

To date, workers have recovered 200,300 litres - or 1,260 barrels - of crude, but the effort has been slowed in recent days by wind-chilled temperatures that dipped to below -56 Celsius.