The US administration has banned oil drilling in new areas of the US coast while the cause of the oil spill off Louisiana is investigated.
White House adviser David Axelrod told ABC TV it wanted to know exactly what led to last week’s explosion on the BP-operated rig in the Gulf of Mexico.
As many as 5,000 barrels of oil a day are thought to be spilling into the water, threatening US coastal areas.
Florida Governor Charlie Crist declared a state of emergency on Friday.
The order, which covers Escambia, Santa Rosa, Okaloosa, Walton, Bay and Gulf counties, says the oil slick “is generally moving in a northerly direction and threatens Florida’s coast”.
Louisiana Governor Bobby Jindal has already declared a state of emergency and asked for federal funds to deploy 6,000 National Guard soldiers to help with the clean-up.
The slick has begun to reach the Louisiana shore and the US Navy has been sent to the area.
Mr Axelrod announced the ban on drilling in new areas in an interview with ABC’s Good Morning America programme.
He also defended the administration’s response to the 20 April explosion that destroyed the Deepwater Horizon rig, saying: “We had the coast guard in almost immediately.”
Last month President Barack Obama eased a moratorium on new offshore drilling.
In a statement outside the White House on Friday, President Obama said he expected reports from the affected area later.
He said he believed oil exploration was an important part of the US economy but it had to be done responsibly.
“BP is ultimately responsible… for paying the costs of response and clean-up operations but we are fully prepared to meet our responsibilities in all affected communities,” he said.
He said five staging areas had been set up to protect sensitive shorelines and about 1,900 emergency workers and more than 300 ships and aircraft were on the scene.
We’re putting everything forth in plans for a worst-case scenario
Cdr Mark McCadden, US Coast Guard
He added that he had ordered a “thorough review” of what might be required “to prevent accidents like this from happening again”.
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.
The wetlands off the Louisiana coast sustain hundreds of wildlife species and a big seafood and fishing industry.
The US Coast Guard said it had sent investigators to confirm whether crude oil had begun to wash up on parts of the Louisiana shoreline.
Cdr Mark McCadden, of the coast guard, told the BBC: “We’re putting everything forth in plans for a worst-case scenario,” he said.
“We can always ramp back on some of those resources but right now the priority is to bring as many resources as are available to attack this spill and try to minimise the effects to the coast and to the public.”
Two US Air Force planes have been sent to Mississippi in case they are needed to spray oil-dispersing chemicals over the slick.
The Louisiana coastline, with its rich shrimp and oyster beds, is the most threatened by the spill.
A group of Louisiana shrimpers has already filed a lawsuit against BP and the owners of the rig, Transocean.
Richard Arsenault, a lawyer for the group, told the BBC he was surprised that such a modern rig couldn’t prevent the spill.
“This is a rig that is valued at some $700m. It’s state of the art… and it is just incredible that with that kind of technology this kind of problem… was not prevented.”
He added: “The harm right now to the fishing industry and to the economic sector is just almost incalculable.”
There are also fears of severe damage to fisheries and wildlife in Mississippi, Alabama and Florida as oil continues to escape from the wreckage of the rig.
An emergency shrimping season was opened on Thursday to allow fishermen to bring in their catch before it was fouled by the advancing oil.
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”.
Eleven workers are still missing, presumed dead, after the Deepwater Horizon rig exploded.
BP’s chief operating officer of exploration and production, Doug Suttles, said the company was using remotely operated vehicles (ROVs) to try to find out how much oil was leaking into the sea.
Mr Suttles put the oil leakage at between 1,000 and 5,000 barrels a day.