BP's runaway well has been sealed, but US officials said Friday they are moving ahead with plans to make sure it's truly "killed" by pumping cement in through a relief well under the Gulf of Mexico.
"We're very close to having the well secured," US spill chief Thad Allen said.
Pressure tests showed that the well no longer has "direct communication with the reservoir" thanks to a top kill operation which pumped drilling mud and cement down through the wellhead, Allen said.
"The well might be effectively killed and we just don't know it," Allen said, adding that he would not be satisfied until he was certain oil would never leak out of the well again.
About 1,000 barrels of oil are believed to be trapped in the well's annulus -- the space between the inner well tubing and the outer casing -- and it's possible that some of the cement pumped in from above effectively sealed the annulus off from the surface.
But to be sure, and reflecting the White House's consistent line that the relief well was the ultimate solution, a decision was made to continue with the so-called "bottom kill" operation.
"Everyone is in agreement that we need to proceed with the relief well. The question is how to do that," Allen told reporters.
One possibility could be to install a new blowout preventer on top of the capped well to make sure oil doesn't escape if a mistake is made during the bottom kill operation, Allen said.
He did not provide an estimated date for completion but said it will take about 96 hours to finish the bottom kill, which had been delayed for a few days because of an approaching storm, once operations begin.
The well ruptured when the BP-leased Deepwater Horizon rig sank on April 22, two days after a massive explosion that killed 11 workers.
At 4.9 million barrels -- enough oil to fill 311 Olympic-sized swimming pools -- the disaster is the biggest maritime spill on record.
It threatened the fish and wildlife-rich US Gulf Coast with environmental ruin and plunged residents of coastal communities into months of anguish over their livelihoods and the region's future.
After a series of failures, BP was able to fully stem the flow of oil into the sea on July 15 with a temporary cap. The well was plugged with cement on August 5.
Explore further: Preparations for BP well 'kill' operation move ahead