Earlier BP expressed confidence that a new cap placed over the ruptured well in the Gulf would capture most of the leaking oil.

But it said it could take 48 hours to know if the system was stable.

Mr Obama has been briefed in Louisiana by Adm Thad Allen, the US official leading the disaster response.

The president's third trip to the Gulf came as beaches in the key tourist area of north-west Florida saw their first major signs of oil.

Mr Obama has for the second time postponed a trip to Australia, Indonesia and Guam in order to deal with the crisis.

Speaking earlier on Friday, Adm Allen said rough estimates showed that 1,000 barrels a day were being captured through the containment cap.

This is only a small part of the 12,000-19,000 barrels a day believed to be leaking.

BP chief operating officer Doug Suttles said he was confident the cap would work and capture up to 90% the spill.

BP says it has spent more than US$1billion so far on the operation to contain the spill and clean up the oil.

Meanwhile, the area affected by the oil slick continues to spread.

Tar balls have now begun to wash up on the beaches of Alabama and north-west Florida.

At Pensacola Beach in Florida, swimmers encountered an oil sheen and children picked up tar blobs as big as tennis balls.