Efforts to siphon off the oil gushing into the Gulf of Mexico showed the first signs of success on Sunday, but officials said it would take months to tackle the pollution.
Coast Guard Admiral Thad Allen said 1,500 tonnes of oil a day were now being captured by a new containment dome.
BP chief executive Tony Hayward said that is "the majority" of the oil leaking per day from a well head that ruptured when the Deepwater Horizon rig exploded April 20, killing 11 workers and causing the largest oil spill in US history.
However, Allen told CBS' Face the Nation: "This is a siege across the entire Gulf. There will be oil out there for months to come. This will be well into the fall."
Allen, who is supervising the US response to the ecological disaster, initially said that some 6,000 barrels of oil were captured in the first 24 hours after BP lowered the dome and began pumping oil to a ship on the surface.
Estimates say anywhere from 1,600 to 3,400 tonnes of oil are gushing out of the ruptured well head every day.
"This is a siege that is going to go on for a long time. We are spread from south-central Louisiana over to Port Saint Joe, Florida. It is not going to end soon, and we need to have our shoulder to the wheel, do everything we can. This is a very, very, very tough problem," Allen said.
"This spill is holding everybody hostage, not only economically but physically. And it has to be attacked on all fronts," he added.
The Gulf winds are now moving the oil closer to the coasts of Mississippi, Alabama and Florida, Allen said, adding that tar balls were seen in areas from western Mississippi to Pensacola, Florida.
While crews in Florida were already removing oil from the state's northwest panhandle region, more oil is expected to arrive in the area within the next three days, according to forecasts from the National Oceanic and Atmospheric Administration.
A live underwater video showed some oil continuing to leak into the ocean. BP technicians hope to stop the flow by closing valves on the "top cap" containment dome in the coming days.
That will lead to increased pressure, which could rip off the dome altogether if it grows too strong.
The risky move has led BP to come up with another solution. Hayward said the company is working on a second, even more efficient, containment dome to be put in place by the end of the month, the New York Times reported on Sunday.
The new containment system will also be designed to be hurricane-proof, which the current dome is not.
But officials have stressed that the containment cap is only a temporary solution. Two relief wells are being drilled that could permanently seal off the underwater leak, but these will not be finished until August.
As of Sunday, an estimated 17,500 National Guard troops from the Gulf Coast states are responding to the spill. And more than 20,000 personnel are working to protect the shoreline and wildlife and clean up coastlines.
Approximately 15.5 million gallons of an oil-water mix have been recovered. And about 1.08 million gallons of total dispersant have been deployed.
So far, 125 controlled burns have been conducted, removing more than 3.2 million gallons of oil from the water in an effort to protect shoreline and wildlife.