Homeland Security Secretary Napolitano: This is a spill of national significance
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.
Up to 5,000 barrels of oil a day may be now be leaking into the water after last week's explosion on a BP-operated rig, which then sank.
The US Coast Guard says oil is expected to start washing ashore on Friday.
A state of emergency has been declared in the state of Louisiana, whose coastline is the most threatened by the spill.
Andy Gallacher, BBC News, Venice, Louisiana
Crews here are now battling against a weather front that is pushing the slick ever closer to the coastline.
While officials believe they can get this huge slick under control, environmentalists are deeply concerned, with some experts predicting a disaster on a huge scale spanning three or four American states.
The US military has now been deployed to help contain this slick, and Louisiana's governor says he'll take all the help he can get.
People here are now bracing themselves for what could be one of America's worst environmental disasters.
While investigators say they still don't know what caused the explosion, officials may now focus on a safety valve that may have failed to stop the flow of oil.
Ms Napolitano is to go to the area to oversee operations along with senior officials from the Department of the Interior and the Environental Protection Agency.
She told a White House briefing: "Today I will be designating that this is a spill of national significance. What that means is that we can now draw down assets from across the country, other coastal areas... (and) that we will have centralised communications because the spill is now crossing different regions."
Later, President Barack Obama said "every single available resource" of government, including the military, would be used to help with the oil spill.
"The entire US government is doing everything possible, not just to respond to this incident, but also to determine its cause," he said.
Meanwhile, the government has ordered inspections of all deep-water oil wells in the Gulf of Mexico to see if anti-spill regulations are being followed.
The US Coast Guard said earlier that up to 5,000 barrels of oil a day - five times the previous estimate - might now be leaking into the water.
BP's chief operating officer of exploration and production, Doug Suttles, 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."
How the oil has spread Approximate oil locations 22 April - 15 May
Mr Suttles estimated something between 1,000 and 5,000 barrels a day was leaking.
A resident of Bay Saint Louis in Mississippi, John Gerger, told the BBC the smell of oil was becoming stronger along the Gulf Coast.
"It's as though a diesel truck is parked in the front yard," he said.
"The potential impact of the slick could be devastating on an area that has just recovered from [Hurricane] Katrina.
"Fishing and shrimping is such an important industry here, and could take a massive hit.
"Local fishermen have been advised to go out and try to recover as much as they can before the slick approaches land."
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.
OIL SPILL DISASTERS
1991: 520m gallons were deliberately released from Iraqi oil tankers during the first Gulf War to impede the US invasion
1979: 140m gallons were spilt over nine months after a well blow-out in the Bay of Campeche off Mexico's coast
1979: 90m gallons leaked from a Greek oil tanker after it collided with another ship off the coast of Trinidad
1983: 80m gallons leaked into the Gulf over several months after a tanker collided with a drilling platform
1989: 11m gallons were spilt into Alaska's Prince William Sound in the Exxon Valdez disaster
A "controlled burn" of surface oil took place in an area about 30 miles (50km) east of the Mississippi River delta.
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.
Oil skimmers, tugboats, barges, special recovery boats that separate oil from water and aeroplanes are working to contain the spill and spray dispersants.
Eleven workers are missing and presumed dead after last week's explosion.
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 (£3.3m) in damages plus undetermined punitive damages.
Under US law, BP will be expected to meet all the costs of the spill clean-up operation.
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