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Bad weather hampered efforts to tackle the huge Gulf of Mexico oil slick, with high seas and winds keeping boats and planes away from the clean-up site.Reports suggest that the slick is growing rapidly - one report said it had tripled in size in a day.
Sheen from the spill has begun washing up on the Louisiana coast, fuelling fears of environmental disaster.
President Obama is due there on Sunday as is BP head Tony Hayward who has been criticised over the BP response.
Up to 5,000 barrels of oil a day are gushing into the sea after the British Petroleum-operated Deepwater Horizon rig exploded and sank last week.
Mississippi, Alabama, Louisiana and Florida have all declared a state of emergency, and analysts say the spill could rival the 1989 Exxon Valdez disaster as the worst in US history.
High winds
Worsening weather conditions have been hampering efforts to contain the slick, now more than 130 miles (200km) long.
Choppy seas meant that smaller boats - contractors and fishermen - that could have helped with the clean up were unable to go out.
Military planes deployed to spray oil-dispersing chemicals have also been grounded.
Rescue groups have been receiving their first patients - seabirds coated in oil - but a BBC correspondent in the area says that at the moment the wind is keeping most of the oil offshore.
The high winds are also forcing some of it over booms meant to contain it, however, and forecasters say that strong winds on Sunday could push more oil onto the Louisiana shore.
Wetlands off the Louisiana coast sustain hundreds of wildlife species, and a major seafood and fishing industry.
The president of Louisiana's Plaquemines Parish, 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 slick 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.
Insurance fears
On Friday Homeland Security Secretary Janet Napolitano called on BP to commit more resources to tackling the catastrophe.
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.
The BBC's 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.
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