Homeland Security Secretary Napolitano: This is a spill of national significance
The US Coast Guard is investigating reports that oil has started washing ashore on the Gulf Coast from a leaking offshore well.
Up to 5,000 barrels of oil a day are thought to be spilling into the water after last week's explosion on a BP-operated rig, which then sank.
President Barack Obama has pledged "every single available resource" to help.
The US navy has been deployed to help avert a looming environmental disaster.
The Coast Guard said it had sent investigators to confirm whether crude oil had begun to wash up on parts of the Louisiana shoreline.
President Obama's administration has banned oil drilling in new areas of the US coast pending investigations into the cause of the oil spill off Louisiana.
"No additional drilling has been authorised and none will until we find out what happened," White House adviser David Axelrod told ABC television.
Last month President Barack Obama eased a moratorium on new offshore drilling.
David Kennedy of the National Oceanic and Atmospheric Administration described the oil spill as a very, very big thing."
He told the Associated Press news agency that the clean-up efforts could be "mind-boggling".
An emergency shrimping season was opened on Thursday to allow fishermen to bring in their catch before it was fouled by the advancing oil.
How the oil has spread Approximate oil locations 22 April - 15 May
The US government has designated the Gulf of Mexico oil spill as an "incident of national significance". This allows it to draw on resources from across the country.
Louisiana Governor Bobby Jindal has declared a state of emergency and asked for federal funds to deploy 6,000 National Guard soldiers to help with the clean-up.
The Louisiana coastline, with its rich shrimp and oyster beds, is the most threatened by the spill. There are also fears of severe damage to fisheries and wildlife in Mississippi, Alabama and Florida.
Navy vessels are helping to deploy booms to contain the spill.
President Obama has dispatched high-level administration officials, including Homeland Security Secretary Janet Napolitano, to the area.
He said they would "ensure that BP and the entire US government is doing everything possible, not just to respond to this incident, but also to determine its cause".
Speaking at the White House, Mr Obama also said: "While BP is ultimately responsible for funding the cost of response and clean-up operations, my administration will continue to use every single available resource at our disposal, including potentially the Department of Defence, to address the incident."
Meanwhile, White House spokesman Robert Gibbs said there was no reason so far to reconsider the president's recent decision to support plans for increased offshore drilling.
"Our focus right now is: one, the area, the spill; and two, also to ultimately determine the cause of it and see the impact that that ultimately may or may not have," Mr Gibbs said.
Eleven workers are still missing, presumed dead, after the Deepwater Horizon rig exploded on 20 April.
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.
MAJOR OIL SPILLS
April 2010: Blast at BP's Deepwater Horizon rig causes crude to spill at rate of up to 5,000 barrels (210,000 US gallons) a day
1991: 520m gallons deliberately released from Iraqi oil tankers to impede US invasion
1989: 11m gallons spilt into Alaska's Prince William Sound in the Exxon Valdez disaster
1983: 80m gallons split off Iran over months after a tanker collides with drilling platform
1979: 140m gallons leak over nine months after a well explosion off Mexico's coast
1979: 90m gallons leak from a Greek oil tanker after collision with another ship off Trinidad
"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 were 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."
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.
Wednesday saw a US Coast Guard crew set fire to part of the oil slick in an attempt to save environmentally-sensitive wetlands.
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.
Under US law, BP will be expected to meet all the costs of the spill clean-up operation.
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