The president of Louisiana's Plaquemines Parish area, Bill Nungesser, said that the oil could cause enormous damage.
"We need to have something out there," he said. "Once it gets into the marsh it is too late. Once it gets behind these islands, through these little canals, you will never clean it up."
Reports suggest that the slick is growing rapidly. Experts from the University of Miami said that the slick had tripled in a day, citing satellite images.
Two natural gas platforms in the Gulf of Mexico have halted production because of the spill and one of them has been evacuated as a safety precaution.
The command centre co-ordinating the response said that the percentage of gas production affected was less than one-tenth of one per cent of the Gulf of Mexico's daily total.
On Friday, Homeland Security Secretary Janet Napolitano called on BP to commit more resources to tackling the catastrophe.
By Joe Lynam, BBC business correspondent
BP's shareholders would probably prefer a few years out of the limelight after a five-year period in which the firm's reputation has been blown about. The fire at a Texas City refinery, the leak of an Alaskan gas pipeline and the sudden departure of former boss Lord Browne, have all been PR disasters.
But this Gulf of Mexico leak may surpass all of them. Apart from the environmental catastrophe, £10bn ($15bn) was wiped off the value of Britain's third-largest company this week.
And now we learn that BP is not even covered by any external insurance and will have to meet compensation claims from its own resources - at a cost of billions of dollars.
And that does not even put a price on the damage to the reputation of a company which describes itself as "Beyond Petroleum".
The British oil giant says it has begun using dispersants underwater in an attempt to break up the leaking oil at its source.
It has dispatched remotely operated vehicles to try to shut off an underwater valve, so far without success. It is also having a relief-well drilled to slow the leak, though experts say that could take up to three months.
In a statement, Mr Obama said BP was "ultimately responsible... for paying the costs of response and clean-up operations".
The president said he had asked Interior Secretary Ken Salazar to conduct a "thorough review" of the catastrophe and report back in 30 days on ways to prevent a repeat.
BP spokeswoman Sheila William told AFP news agency the energy firm was prepared to assume costs for the clean-up and for damages.
BBC business correspondent Joe Lynam says that BP has no external insurance cover in the traditional sense, instead using a form of "self insurance" to cover major events like this.
The company would therefore have to cover the full cost of any legitimate compensation claims from the oil spill from its own resources.
The cause of last week's blast, which left 11 workers missing, presumed dead, remains unclear.
But it has emerged that BP last year downplayed the possibility of such a disaster at the offshore rig.
In BP's 2009 exploration plan for the well, the firm suggested an oil spill was unlikely or virtually impossible, AP news agency reports.
How the oil has spread Approximate oil locations 22 April - 15 May
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