BP was desperately wrestling a cap into place late Thursday over a ruptured deep-water well gushing crude into the Gulf of Mexico, as President Barack Obama said he was "furious" about the disaster.
Live video feed provided by BP showed the inverted, funnel-like cap being placed on the well's fractured riser pipe in near-freezing waters, nearly a mile (1,600 meters) below the surface. BP intends to then siphon the oil to a ship on the surface.
Dark clouds of oil and gas continued to spew out unimpeded with great force, complicating efforts to determine whether the cap was in fact a good fit. BP and US Coast Guard officials did not immediately respond to requests for comments.
"Placement of the containment cap is another positive development in BP's most recent attempt to contain the leak," Lieutenant Commander Tony Russell, spokesman for Coast Guard Commandant Admiral Thad Allen, said in a statement provided to CNN.
"However, it will be some time before we can confirm this method will work and to what extent it will mitigate the release of oil into the ocean."
Engineers have already acknowledged that the cap will not be a fix-all, and some of the crude will still leak out even if it is successfully placed over the gusher.
BP earlier managed to slice off the fractured well pipe with a pair of giant shears, but the cut was jagged and officials had to resort to a looser-fitting cap.
The British energy giant's chief executive, Tony Hayward, has said it could take about a day after the cap is put in place to know if it is managing to contain the worst of the spill, amid warnings that, with the broken pipe cut off, the oil flow would initially increase by up to 20 percent.
More than six weeks into the disaster, the federal government sent BP a 69-million-dollar bill to reimburse American taxpayers for the government's costs so far in battling the worst oil spill in US history, with thousands of barrels of oil pouring into the Gulf every day.
On the eve of his third visit to the region on Friday, Obama said "yelling" would be pointless as aides rejected claims the president had shown insufficient emotion over the crisis.
He also disputed a media narrative that political trauma over America's worst environmental catastrophe would crimp his broader political agenda.
In a gripping illustration of the slick's impact on the fragile coastline, helpless birds could be seen writhing in thick oil along Louisiana's coast, graphic images likely to sharpen anguish after BP's repeated failures to stop the flow of oil.
"I would love to just spend a lot of my time venting and yelling at people, but that is not what I was hired to do - my job is to solve this problem," Obama told CNN, adding he was "furious at this entire situation".
At the start of his estimated five-hour trip, the president is due to meet with Allen, who is overseeing the federal response, as well as state and local elected officials.
After the cap, the next chance to halt the flow of oil would not come until mid-August, when two relief wells are due to be completed.
"It's just the beginning. It's been an extraordinary endeavor on the part of many, many people," Hayward said.
BP has battled unsuccessfully to cap or contain the disastrous leak since an April 20 explosion tore through the BP-leased Deepwater Horizon rig off the Louisiana coast.
Hayward acknowledged earlier in an interview with the Financial Times that the British energy giant had been unprepared for the disaster and "did not have the tools you would want in your tool kit."
The US government has estimated the flow of oil before the riser was cut away at 12,000 to 19,000 barrels a day - meaning between 22 million and some 36 million gallons have already poured into the Gulf.
By comparison, Alaska's 1989 Exxon Valdez disaster - the worst US oil spill before the Deepwater Horizon blast - resulted in an 11-million-gallon spill.
The slick, spreading in a myriad of oily ribbons, is now threatening Alabama, Mississippi and Florida after contaminating more than 125 miles (200 kilometers) of Louisiana coastline.
With the slick expected to reach the Florida panhandle within days, Coast Guard investigators were also probing unconfirmed reports that tar balls and an oily substance had been found in the tourist-heavy Florida Keys.
Allen, who is coordinating the government's response to the spill, said nearly a million gallons of dispersants have been used to break up the oil in the Gulf.
"It's a milestone and there are concerns about that and we will continue to work the dispersants very, very closely."
Many environmentalists have voiced concern over the unprecedented use of so much dispersant, warning its long-term effect on wildlife - already threatened by the huge oil spill - is unknown.
Experts also warn the majority of the oil is contained in vast underwater plumes that cannot be measured from above.