• A boat uses a protective boom to collect oil leaking from the wrecked Deepwater Horizon rig off the coast of Louisiana. Picture: Getty
The Coast Guard said five times more oil than previously estimated was spilling into the ocean leaking from the well beneath where a BP-operated rig exploded and sank last week.
And deputy interior secretary David Hayes warned up to 5,000 barrels of oil a day could be lost for 90 days before the leak could be stopped.
The slick, first thought to pose little threat, could destroy fragile marshlands and fisheries and pollute the Mississippi River.
The US government pledged an all-out response and dispatched top officials to the region to help co-ordinate defences against the potential environmental disaster.
"We are being very aggressive and are prepared for the worst case," Coast Guard Rear Admiral Sally Brice O'Hare said at the White House.
The spill was "of national significance", Janet Napolitano, secretary of the Department of Homeland Security, said, adding that the government would push BP – which operated the rig that exploded and sank 40 miles offshore last week – to conduct the strongest possible effort to clean it up.
Three members of president Barack Obama's cabinet – Ms Napolitano, interior secretary Ken Salazar, and environmental protection administrator Lisa Jackson – were travelling to the scene to oversee operations.
The spill could delay Mr Obama's new deepwater oil drilling plan, with an interior department official stating that the government would consider delaying the programme until companies showed they could control oil spills.
Federal officials announced that inspections would begin immediately of all oil rigs in the Gulf of Mexico.
However, they said the priority was to support BP in employing booms, skimmers, chemical dispersants and control burns to fight the oil surging from the seabed.
Officials emphasised that all costs of the defence and recovery operation would ultimately fall on the industry, not taxpayers.
Last night, oil had crept to within 12 miles of the coast, which it could hit today. A third leak has been discovered, which US officials said is spewing five times as much oil into the sea as first estimated – about 5,000 barrels a day.
BP chief operating officer Doug Suttles initially disputed that estimate, or that BP was unable to handle the operation to contain it. But yesterday, he acknowledged the leak may be as bad as the government said.
If the well cannot be closed, almost 100,000 barrels of oil, or 4.2 million gallons, could spill into the Gulf of Mexico before crews can drill a relief well to ease the flow.
The tanker Exxon Valdez leaked 11 million gallons into Alaska's Prince William Sound in 1989, the worst oil spill in US history.
The Deepwater Horizon rig sank a week ago after exploding two days earlier. Eleven of its 126 crew are missing presumed dead. It was owned by Transocean and operated by BP.