BP claimed a key victory on Wednesday in its effort to plug its blown-out oil well in the Gulf of Mexico while the government said the vast majority of oil from the worst offshore spill in US history was already gone.
Declaring it a milestone, BP PLC said mud that was forced down the well was holding back the flow of crude and it was in a "static condition."
Also, White House energy adviser Carol Browner said on morning TV talk shows that a new assessment found that about 75 per cent of the oil has either been captured, burned off, evaporated or broken down in the Gulf.
"It was captured. It was skimmed. It was burned. It was contained. Mother Nature did her part," Browner told NBC television.
On ABC television, she said about 25 per cent remained. In the Gulf, workers stopped pumping mud in after about eight hours of their "static kill" procedure and were monitoring the well to ensure it remained stable, BP said.
"It's a milestone," BP PLC spokeswoman Sheila Williams said.
"It's a step toward the killing of the well."
The next step would be deciding whether to cement the well, Williams said.
The pressure in the well dropped quickly in the first 90 minutes of the static kill procedure on Tuesday, a sign that everything was going as planned, wellsite leader Bobby Bolton told The Associated Press.
Bolton said Tuesday night that the procedure was going well.
"Pressure is down and appears to be stabilizing," he told the AP then.
Browner told NBC it was good news that the static kill was working but that "we remain focused on the relief well."
The static kill - also known as bullheading - involved slowly pumping the mud from a ship down lines running to the top of the ruptured well a mile below. BP has said that may be enough by itself to seal the well.
But the mud that was forced down the broken wellhead to permanently plug the gusher is only half the story. To call the mission a success, crews working on a flotilla of vessels on a desolate patch of water need to seal off the well from two directions.
An 18,000-foot (5,500-meter) relief well BP has been drilling for the past three months will be used later this month to execute a "bottom kill," in which mud and cement will be injected into the bedrock 2 miles (4 kilometers) below the sea floor to finish the job, retired Coast Guard Adm. Thad Allen said.
A 75-ton cap placed on the well in July has been keeping the oil bottled up inside over the past three weeks, but is considered only a temporary measure. BP and the Coast Guard want to plug up the hole with a column of heavy drilling mud and cement to seal it off more securely.
Before the cap was lowered onto the well, 172 million gallons (651 million liters) of crude flowed into the sea, unleashed by the April 20 explosion aboard the drilling rig Deepwater Horizon that killed 11 workers. A previous, similar effort failed in May when the mud couldn't overcome the unstemmed flow of oil.
BP won't know for certain whether the static kill has succeeded until engineers can use the soon-to-be-completed relief well to check their work.
The task is becoming more urgent because peak hurricane season is just around the corner, Allen said.
Tropical Storm Colin formed then dissipated far out in the Atlantic on Tuesday, but early forecasts say it will travel toward the East Coast rather than the Gulf.