An oil spill discovered at Prudhoe Bay field is the largest ever on Alaska's North Slope region, US officials say.
The spill covers two acres of the snow-covered tundra
They estimate that up to 267,000 gallons (one million litres) of crude leaked from a corroded transit pipeline at the state's northern tip.
The spill was detected on 2 March and plugged. Local environmentalists have described it as "a catastrophe".
In 1989, the Exxon Valdez shipping disaster spilled 11m gallons (42m litres) of oil onto the Alaskan coast.
"I can confirm it's the largest spill of crude oil on the North Slope that we have record of," Linda Giguere, from Alaska's state department of environmental conservation, was quoted as saying by the Associated Press news agency.
The estimate is based on a survey conducted several days ago at the site where the leak was discovered, officials say.
The spill covers about two acres (one hectare) of the snow-covered tundra in the sparsely populated region on Alaska's north coast, some 1,040km (650 miles) north of the state's biggest city, Anchorage.
The source of the spill was a hole caused by internal corrosion in the pipeline, officials say. It remains unclear when the leak started.
Environmentalists from Alaska Wilderness League said the spill was "a catastrophe for the environment".
They said it was "a painful reminder of the reality of unchecked oil and gas development across Alaska's North Slope".
They also urged lawmakers to shelve a Republican-led project to allow drilling for oil in Alaska's Arctic National Wildlife Refuge (ANWR).
Supporters of drilling in Alaska say it offers an alternative source of energy to the Middle East and so would improve national security.
Opponents warn oil exploration would harm a pristine wilderness and endanger a key habitat for migratory birds, polar bears, caribou and other animals.
Alaska's worst-ever oil spill happened on 24 March 1989.
The 1989 spill devastated miles of Alaskan coastline and wildlife
The Exxon Valdez tanker ran aground in Prince William Sound, near Anchorage, contaminating around 1,300 miles (2,080km) of coastline.
Its captain, Joseph Hazelwood, admitted drinking vodka before boarding the vessel, but was subsequently acquitted of operating a ship while intoxicated.
The spill killed an estimated 250,000 seabirds, 2,800 sea otters, 300 seals, 250 bald eagles, up to 22 Orca or killer whales, and an unknown number of salmon and herring.
In 2004, a federal judge in Alaska ordered Exxon to pay $6.75bn (£3.9bn) in damages and interest in relation to the spill.