Three US states have now declared a state of emergency as the expanding slick from a sunken oil rig in the Gulf of Mexico approaches the country's southern coastline.
The growing alert comes as forecasters predict worsening weather and unusually high tides over the next few days, hampering efforts to contain the spill.
Recovery workers have been deploying miles of inflatable booms hoping to contain the oil, but strong winds and heavy seas have caused several of the barriers to break loose and washed them onto the shore.
Meanwhile it has emerged that the company operating the rig, the British energy company BP, had no contingency plan for a spill in the Gulf of Mexico.
A report commissioned by the company had concluded that it was unlikely a spill would occur and that an ecological disaster was virtually impossible.
Authorities in the states of Alabama joined Louisiana and Florida in declaring an emergency on Friday, mobilising thousands of personnel to try to clean up the oil and protect vulnerable fishing grounds and wildlife habitats.
Growing oil disaster puts fragile coastal wildlife habitats under threat
The US administration meanwhile has labelled the spill a disaster of "national significance", a step which paves the way for the better co-ordination of clean-up resources and personnel.
In Louisiana the state governor has received approval to deploy at least 6,000 US National Guard troops to help with recovery operations.
At the same time the US air force is preparing to spray oil-dispersing chemicals from C-130 cargo jets flying off the Louisiana coast.
Thousands of barrels of oil a day have been leaking into the Gulf following last week's sinking of the Deepwater Horizon rig.
The rig, operated by oil giant BP, sank after an explosion and fire which left 11 oil workers missing, presumed dead.
The cause of the explosion is not clear.
New drilling halted
Barack Obama, the US president, ordered a halt on Friday on any new drilling projects until an investigation and into the causes of the disaster.
The drilling freeze is a largely symbolic gesture as no new leases had been scheduled for the coming months, but Obama said there would also be a "thorough review" of existing off shore drilling operations.
He said in Washington on Friday that the government had dispatched teams to the coast "to inspect all deep water rigs and platforms to address safety concerns".
Obama said he had ordered officials to produce a report within 30 days on precautions required to prevent a recurrence of such a disaster.
With efforts to plug the leak so far unsuccessful, fears are growing of an ecological disaster that could rival the 1989 Exxon Valdez spill in Alaska.
Already the first birds coated in oil have been received by animal rescue centres, and wildlife groups are braced to receive many more casualties.
Fishing boat operators in the area have been trying to get as much of their catch as they can before the oil slick hits the shoreline.
"People have been hearing that they have about one or two days left to fish before the whole operation is shut down," Al Jazeera's Sebastian Walker, reporting from a fishing port south of New Orleans, said.
|Several booms meant to contain the oil have been washed on shore [AFP]
"There's an atmosphere of extreme concern here, with fishermen trying to get as much as they can before things get even worse."
With fears growing of a massive ecological and economic disaster state and federal officials have been stepping up pressure on BP, the operator of the sunken rig, to do more to contain the spill.
"I do have concerns that BP's resources are not adequate," Bobby Jindal, the Louisiana governor, said at a press briefing.
"I urge them to seek more help from the federal government and others."
Obama meanwhile said in Washington that as the operator of the Deepwater Horizon rig, BP was "ultimately responsible... for paying the costs of response and clean-up operations".
Pressure on BP
Obama's comments came as Janet Napolitano, the US homeland security secretary, flew to Louisiana to press BP to use all its resources to aid the clean-up effort.
"We will continue to push BP to engage in the strongest possible response," she said.
|Animal rescue centres have begun to see the first birds affected by the oil [Reuters]
Ken Salazar, the US interior secretary, who was travelling with Napolitano, said he had pressed the chief executive of BP to "work harder and faster and smarter to get the job done".
The US government, he said, would not rest until BP seals the well and "they clean up every drop of oil".
The British energy giant has said it is "taking full responsibility" for the oil spill and will pay for "legitimate claims" stemming from the disaster.
At least two lawsuits have already been filed on behalf of fisherman in the Gulf area, in what is expected to be a flood of litigation from the disaster.
The region is a prime spawning ground for fish, shrimp and crabs, home to oyster beds and contains large tracts of wetlands that form a major stop for migratory birds.
"For birds, the timing could not be worse; they are breeding, nesting and especially vulnerable in many of the places where the oil could come ashore," Melanie Driscoll of the Audubon Society, a nature conservancy group, said.