After failing again to stem the flow of oil into the Gulf of Mexico, BP scrambled to control the spill, which has been described by the president's top energy adviser as the biggest environmental disaster in US history.
Hours after the British oil giant acknowledged failure in its "top kill" attempt to plug the underwater well, company officials said it could take a week to implement the next bid - placing a cap over the leak.
"Right now we are going to a containment operation," BP Managing Director Bob Dudley told CNN's State of the Union program of the latest attempt to deal with the ruptured well about 1.5 kilometres under water.
"Because this is being done at 5,000 feet with robots, we're going to take our time, do it extremely carefully. By the end of the week, we should have this in place," Dudley said.
While the "top kill" would have sealed the well using a combination of heavy drilling fluid and eventually cement, the new effort aims only to contain most of the leak, and might temporarily increase the amount of oil gushing into the Gulf of Mexico, officials said.
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The well will only be sealed when BP finishes drilling two relief wells - which are supposed to be a better long-term solution - but those are not expected to be ready until August.
That means the relief well won't be completed until the middle of the Atlantic hurricane season, which begins on Tuesday.
President Barack Obama's top energy advisor, Carol Browner, said on Sunday that the spill was "probably the biggest environmental disaster we've ever faced in this country."
"I think what the American people need to know is that it is possible that we will have oil leaking from this well until August when the relief wells will be finished," she said.
The spill has dumped between 18 million gallons (68 million litres) and 40 million gallons (150 million litres) into the Gulf, according to government estimates.
The leak began after the Deepwater Horizon drilling rig exploded on April 20, killing 11 workers.
Some 500 people braved rains in New Orleans' French Quarter on Sunday to denounce BP for the oil spill, as well as the Obama administration's response to the disaster.
The new bid underway involves using robots to sever a damaged riser pipe carrying oil from the wellhead and placing a containment device called a Lower Marine Riser Package (LMRP) over the leak.
Oil would then be siphoned from the device up to a container ship at the surface. But the process could actually increase the amount of oil leaking into the sea, and it is uncertain how much oil would be contained, Browner said on CBS's Face the Nation on Sunday.
On Friday, Obama visited Louisiana for the second time since the spill began, and he pledged on Saturday to do whatever it takes to help those whose livelihoods have been upended by the catastrophe.
"We will not relent until this leak is contained, until the waters and shores are cleaned up, and until the people unjustly victimised by this manmade disaster are made whole," he said.
Retired US Coast Guard admiral Thad Allen - who is leading the government's response to the spill - said at a briefing with Obama on Sunday that the federal containment effort had redoubled, White House spokesman Robert Gibbs said in a statement.
"Officials on the ground have increased efforts to be more responsive to needs identified by local communities," Allen said, according to Gibbs. Since the spill began, an estimated 12,000 to 19,000 barrels of crude have leaked into the Gulf each day.
The disaster has already closed stretches of coastal fishing waters, endangering the seafood industry and tourism, and threatening a catastrophe for Louisiana marshes, home to many rare species.
Government data released Thursday suggested between 18.6 million gallons and 29.5 million gallons of oil have poured into the Gulf - far more than the roughly 11 million gallons of crude spilled in the 1989 Exxon Valdez disaster.
The spill has been a public relations nightmare for BP, which faced new allegations of negligence on Sunday after The New York Times said internal company files showed the firm had serious concerns about the Deepwater rig weeks before the accident.
