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 kilometers (650 miles) north of Alaska's biggest city, Anchorage.

In its latest report on the incident, the Alaska Department of Environmental Conservation estimated that 760,000 to one million liters (201,000 to 267,000 gallons) of oil have spilled into 0.8 hectare (1.93 acres) of tundra and frozen lake surface.

The previous estimate released last week said 220,000 liters (58,000 gallons) of crude had spewed onto the tundra.

The biggest crude leak on the North Slope until this month's spill had been a 127,500-litre (39,850-gallon) spill in 1989.

The extreme cold weather has hampered the cleanup operation, the environmental department said.

"The priorities are still to vacuum up the oil and collecting the contaminated snow, to make sure the tundra is not contaminated," department spokeswoman Lynda Giguere told AFP.

"The project is going on very smoothly, it's just a matter of having the weather cooperating with us," she said.

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 11 million gallons (41.8 million liters) of crude when it hit a reef at night, unleashing one of the world's worst environmental disasters.

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