US President Barack Obama has said it is "way too early to be optimistic" as he made his third visit to the oil-hit Gulf of Mexico coast.
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 BBC's North America editor Mark Mardell says the political fallout from the spill is growing, and the president's visit is part of efforts to combat criticism of his handling of the situation.
After landing at New Orleans airport, Mr Obama met Adm Allen for a briefing before driving to Grand Isle, a barrier island town hit by the spill.
Speaking to reporters after the briefing, the president said there seemed to be progress but it was too soon to be optimistic about a solution to the spill.
He criticised BP for failing to rule out a quarterly shareholder dividend, saying the company should not be "nickel and diming" Gulf coast residents over damage claims while spending billions on the dividends.
Federal officials would watch to make sure BP lived up to its promises and dealt with residents' claims "quickly and fairly", he said.
Mr Obama also questioned whether BP should be spending a reported $50m on TV advertising to improve its image while the crisis was still going on.
"This has been a disaster for this region, and people are understandably frightened and concerned about what the next few months and the next few years may hold," he added.
White House spokesman Robert Gibbs said Mr Obama had invited the families of the 11 workers killed when the Deepwater Horizon rig exploded and sank in April to visit the White House next Thursday.
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.
Adm Allen said the amount should increase as BP shut off vents to capture more of the oil.
He added: "Progress is being made, but we need to caution against over-optimism."
But 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 $1bn (£686m) so far on the operation to contain the spill and clean up the oil.
In a conference call earlier to investors, BP chief executive Tony Hayward said the firm was "heartbroken" by the loss of life, damage to the environment and impact on livelihoods caused by the spill.
The estimates for the amount of oil that has already leaked vary widely from 20 million to 45 million gallons.
BP had previously made a number of unsuccessful efforts to halt the leak and accepts it may not be fully contained until relief wells now being drilled become ready in August.
Mr Hayward said BP was working hard to rebuild the trust of the American people and to ensure such an event would never happen again.
He admitted that people wanted to know the cause of the disaster.
But he said it was a complex accident caused by an "unprecedented number of failures" and "a lot remains unknown".
The company also said it would not make a decision on its next dividend payment until late July.
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.