The US government has designated the Gulf of Mexico oil spill as an “incident of national significance”.
Homeland Security Secretary Janet Napolitano told reporters that this move would allow resources to be ordered in from other areas of the US.
The US military has joined efforts to stem oil leaking from the well beneath where a rig exploded and sank last week, as fears rise about its scale.
The US Coast Guard says oil is expected to start washing ashore on Friday.
Rear Admiral Mary Landry said 5,000 barrels a day – five times more than initially thought – were believed to be gushing into the sea off Louisiana.
Ms Napolitano is to go to the area to oversee operations.
Andy Gallacher, BBC News, Venice, Louisiana News that up to five times as much oil is now leaking from the sunken rig have given a new sense of urgency to the situation here on the Gulf Coast.
The huge slick is now close to reaching land and if the weather turns as forecasters predict, the oil may reach the coastline sooner than thought.
Attempts at controlled burns may be inevitable but Louisiana is now asking for outside help to try to stem the flow of oil.
There is some dispute as to how much oil is now spewing from the sunken rig but all along this delicate coastline, officials are preparing themselves for what could be a huge environmental disaster.
Meanwhile, the government had ordered inspections of all deep-water oil wells in the Gulf of Mexico to see if anti-spill regulations are being followed.
The oil slick caused by the leak from the BP-operated Deepwater Horizon rig is 45 miles by 105 miles.
If the coastguard estimate is correct, within two months the spill could match the 11 million gallons spilt from the Exxon Valdez tanker off Alaska in 1989.
BP’s chief operating officer of exploration and production, Doug Suttles, welcomed the US military’s offer of help.
He said the company was using remote operative vehicles (ROVs) to try to find out how much oil was leaking into the sea.
“This is very, very difficult to estimate,” Mr Suttles told reporters.
“Down below the surface we actually can’t meter this oil so we can just observe it… what our ROV pictures show to us on the sea floor hasn’t changed since we first saw the leak… but what we can say based on what we’re picking up on the surface it looks like it is more.”
Mr Suttles estimated something between 1,000 and 5,000 barrels a day was leaking.
Meanwhile, a firefighting expert said the disaster may become the biggest oil spill ever.
Mike Miller, head of Canadian oil well firefighting company Safety Boss, told the BBC World Service: “Probably the only thing comparable to this is the Kuwait fires [following the Gulf War in 1991].
“The Exxon Valdez is going to pale in comparison to this as it goes on.”
Scientists say only a quarter of local marine wildlife survived the Exxon Valdez disaster.
The scale of the operation to contain the oil spill and protect both the US coastline and wildlife is unprecedented, with the military and other government agencies collaborating with BP – which had hired the sunken rig – and industry leaders.
Efforts to stem the flow are being complicated by the depth of the leak at the underwater well, which is about 5,000ft (1,525m) beneath the surface.
A coastguard crew has set fire to part of the oil slick in an attempt to save environmentally-fragile wetlands.
A “controlled burn” of surface oil took place in an area about 30 miles (50km) east of the Mississippi River delta.
But Mr Miller warned that burning off leaking oil was not a long-term solution.
“The object of this game is to shut off the flow,” he said.
Engineers are working on a dome-like device to cover oil rising to the surface and pump it to container vessels, but it may be weeks before this is in place.
It is feared that work on sealing the leaking well using robotic submersibles might take months.
BP is also working on a “relief well” to intersect the original well, but this is experimental and could take two to three months to stop the flow.
Seventy vessels – oil skimmers, tugboats barges and special recovery boats that separate oil from water – as well as five aeroplanes, were working to spray dispersants and round up oil, BP said.
Eleven workers are missing and presumed dead after the worst oil rig disaster in almost a decade.
Louisiana’s coast contains some 40% of the nation’s wetlands and spawning grounds for countless fish and birds.
AFP news agency reports that two Louisiana shrimpers have filed a lawsuit accusing the operators of the rig of negligence, and seeking at least $5m in damages plus undetermined punitive damages.
Under US law, BP will be expected to meet all the costs of the spill clean-up operation.
© BBC MMX