The leak, apparently caused by metal corrosion, was detected March 2 in the United States' largest oil field in Prudhoe Bay, which lies about 1,040 kilometres north of Alaska's biggest city, Anchorage.
The Alaska Department of Environmental Conservation estimated that between 760,000 and one million litres of oil have spilled into 0.8 hectares of tundra and frozen lake surface.
An earlier estimate said 220,000 litres of crude had been spilled. The extreme cold weather has hampered the cleanup operation.
"The priorities are still to vacuum up the oil and collecting the contaminated snow, to make sure the tundra is not contaminated," said US environment department spokeswoman Lynda Giguere.
"The project is going on very smoothly, it's just a matter of having the weather cooperating with us," she said.
The biggest crude leak on the North Slope until this month's spill had been a 127,500-litre spill in 1989.
Alaska's worst ever oil spill took place after the Exxon Valdez tanker ran aground in Prince William Sound, south of Anchorage, on March 24, 1989.
The Exxon Valdez was laden with 41.8 million litres of crude when it hit a reef at night, unleashing one of the world's worstenvironmental disasters.