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Speaking in Louisiana, Mr Obama said his government would do whatever it takes to clean up the oil, adding that BP was responsible and must pay.

He said the focus was now on preventing any further damage to the Gulf coast.

BP says it will be at least a week before temporary measures to stem the leak are in place.

But it could take up to three months to drill relief wells that could fully contain the spillage, Interior Secretary Ken Salazar warned on Sunday.

BP is responsible for this leak, BP will be paying the bill

Barack Obama
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The BP-operated Deepwater Horizon rig sank on 22 April, two days after a huge explosion that killed 11 workers.

Louisiana Governor Bobby Jindal has warned the spill threatens the way of life in his state.

Mr Obama flew to Louisiana on Sunday to see for himself the damage.

Speaking in the town of Venice, he said: “We're dealing with a massive and potentially unprecedented environmental disaster.

“The oil that is still leaking from the well could seriously damage the economy and the environment of our Gulf states.

“And it could extend for a long time. It could jeopardise the livelihoods of thousands of Americans who call this place home.”

'Mitigate the damage'
The president said the slick was now nine miles (14km) off the coast of south-eastern Louisiana.

BBC meteorologist Daniel Corbett explains how the weather will affect the slick

And he warned: “BP is responsible for this leak. BP will be paying the bill.”

BP chief executive Tony Hayward, who is in Louisiana to oversee the company's clean-up, said: “I agree with the president that the top priority right now is to stop the leak and mitigate the damage.”

The company has said it will honour legitimate claims for damages.

BP chairman Lamar McKay said they hoped to lower a hastily made dome a mile below the surface to cap the wellhead in the next six to eight days, as a short-term option.

There have been warnings that within weeks the spill, if unchecked, could eclipse the 1989 Exxon Valdez disaster as the worst in US history.

The Louisiana wetlands host a multi-billion-dollar fishing industry and are a prime spawning area for fish, shrimp, crabs and oysters.

Mississippi, Alabama and Florida have also declared a state of emergency and are considering their legal options.