US President Barack Obama says the widening Louisiana oil slick could have a "potentially unprecedented" environmental and economic impact.
Mr Obama was speaking after arriving in Louisiana to see for himself the devastation caused by a leaking undersea well in the Gulf of Mexico.
He says the leak could seriously damage the economy and environment of the Gulf states and it could extend for a long time.
About six million litres of oil have spilled into the sea since an oil rig operated by BP exploded and sank earlier this month
Mr Obama, who will hold talks with officials from five states on how the disaster is being tackled, says BP will be held responsible for it.
No immediate help seen in relief well
BP is drilling a relief well to divert the flow of oil but US Interior Secretary Ken Salazar has said it could be three months before that well is in place and the leak is stopped.
Satellite images show the slick is growing faster than anticipated.
Officials in Louisiana now concede there is no way to prevent the slick from reaching the shoreline and killing wildlife, and Governor Bobby Jindal says the slick threatens the "very way of life" of people in the state.
All fishing is closed for at least 10 days in the affected area.
BP must meet all costs
The leak began after the Deepwater Horizon oil rig sank on 22 April, two days after it exploded and caught fire, killing 11 men.
Under US law, BP must meet all the costs of the clean-up. The company says it will compensate all who are affected.
The company, which describes the situation as an unprecedented emergency, is also trying to activate an underwater blowout valve that could turn off the flow, but so far without success.
