A new steel dome has been placed beside the damaged oil well that has been polluting the Gulf of Mexico since last month’s drilling rig disaster, BP says.
Dubbed the "top hat", it is smaller than a first container dome which had to be set aside after becoming blocked by crystallised gas hydrates.
It is on the seabed but is being kept away from the well for now, BP said.
The cost of tackling the spill is being put at $118m (£80m), much of which the Obama administration expects BP to pay.
The figure is contained in legislation being sent to Congress, which also calls for oil companies to pay a 1% per barrel tax increase to the existing Oil Spill Liability Trust Fund, senior administration official Jeff Liebman said in Washington.
Eleven people died when an explosion – thought to have occurred after a surge of methane gas from deep within the well – destroyed the Deepwater Horizon rig on 20 April.
Some 5,000 barrels (210,000 gallons) of oil a day have been leaking into the sea from the damaged well.
US federal wildlife officials are treating the deaths of six dolphins on the Gulf Coast as oil-related even though other factors may be to blame.
Samples from the carcasses found in Louisiana, Mississippi and Alabama since 2 May have been sent for testing.
A National Marine Fisheries Service official said none of them had obvious signs of oil and it was common for dead dolphins to wash up at this time of year, when they are in shallow waters to calve.
‘Faulty blowout preventer’
The dome is meant to funnel some of the escaping oil to a waiting tanker on the surface.
"The ‘top hat’ was lowered to the seabed floor last night and is presently… in the immediate area of the leak and the plan is to have that positioned over the leak and functioning by the end of the week," BP’s Bryan Ferguson told AFP news agency.
Containment chambers like this have been used to tackle well and pipeline leaks in the past but not at such a depth – 5,000ft (1,525m).
At 4ft (1.2m) in diameter and 5ft (1.5m) in height, it is much smaller than the first, 40ft dome.
US House of Representatives investigators say they have uncovered significant problems with the crucial blowout preventer (BOP) on the well, which may have contributed to the explosion, Reuters news agency reports.
The BOP had a leak in its hydraulic system and lacked the power to cut through joints to seal the drill pipe, Representative Henry Waxman told a hearing on the spill.
The hearing was also told that the BOP had been modified, which made it difficult to operate after the accident, and its emergency back-up controls may have failed because the explosion that destroyed the rig also disabled communications preventing workers from sending signals underwater.
On Tuesday, BP and other oil industry executives traded blame in Congress.
The British oil giant told a Senate hearing that the BOP, made by drilling contractor Transocean, had failed.
Senators heard Transocean argue in turn that BP had been in charge and that a third firm, a BP contractor, did not plug the exploratory well properly.
How the oil has spread
Approximate oil locations 22 April – 12 May
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