The spilled oil spread over 1.9 acres of snow-covered tundra and the environmental impact remains unknown, according to Leslie Pearson, on-scene coordinator for the Department of Environmental Conservation.

The spill was discovered on March 2 on the west side of Prudhoe Bay by a worker for field operator BP Exploration (Alaska) Inc.

Officials expect the cleanup to take four to six weeks. So far, 52,920 gallons (1,260 barrels) of oil had been recovered, according to a news release issued by the state, federal and BP's joint emergency response team.

"Although it is a significant spill and it is a large volume, the footprint, being just under two acres, is small," said Pearson.

Alaska's biggest oil spill was the 11 million-gallon Exxon Valdez disaster of 1989. Crude oil from the grounded Exxon tanker spread to 1,300 miles (2,092 kilometres) of coastline, including sites in the Chugach National Forest and in three national park units.

The Prudhoe Bay spill forced BP to shut down operations at a gathering center, a facility that separates pumped oil from water and natural gas, and at the approximately 230 wells that feed into it.

The shutdown caused BP to lose about 100,000 barrels of daily production, although the company has been able to divert production of about 5,000 of those barrels through a different pipeline, BP spokesman Daren Beaudo said.

The Prudhoe Bay field normally produces about 470,000 barrels a day, a little more than half of all North Slope oil output. Other major owners of the field unit are ConocoPhillips and Exxon Mobil.

"Knowing the impact area and getting it cleaned up, to us, is in a sense the most important thing," said Beaudo, who added that restarting production is less of a priority.

Officials suspect corrosion created a quarter-inch hole in the transit line and the ensuing leak, even though BP said the area of the breach was not registered as vulnerable as part of the company's corrosion-monitoring program.

Opponents of Arctic National Wildlife Refuge oil development said the Prudhoe Bay spill weakens claims that drilling and other operations could be conducted safely.

"By nature, the oil industry is a messy business," said Luci Beach, executive director of the Gwich'in Steering Committee, a group of Athabascan Indians opposed to oil development in the refuge.

Beach, who was in Washington, D.C., to campaign against Congressional attempts to allow drilling in the refuge, said she often encounters arguments that modern oil-field technology has eliminated environmental risks.