Engineers monitored a newly-capped oil well in the Gulf of Mexico on Sunday amid cautious optimism the months-long spill behind the worst environmental disaster in US history has been finally contained.
The tests, which involve multiple pressure readings on the well-bore that runs to the oil reservoir below the seafloor, have provided "valuable information" and will continue on Sunday, Coast Guard Admiral Thad Allen said.
Once the analysis is complete, the British energy firm will open the new cap and resume siphoning off the oil to two production vessels on the sea surface, he added.
BP said earlier on Saturday that the cap placed over the gushing wellhead was still holding back spilling crude, but the results of tests on the well's structure required more analysis.
"We're feeling more confident that we have integrity," BP senior vice president, Mr Kent Wells, told reporters.
The tests began on Thursday after valves on the cap were sealed, choking off the flow of crude into the Gulf for the first time since the spill began in April.
Allen said that pressure in the capping stack was continuing to increase "very slowly and we want to continue to monitor this progress."
"When this test is eventually stopped, we will immediately return to containment, using the new, tighter sealing cap with both the Helix Producer and the Q4000" containment ships, he added.
BP's return to the containment system was expected to see oil flowing back into the sea once again, at least temporarily, as the pressure in the well is lowered and the new system is established.
The oil firm plans to eventually reach a total collection capacity of up to 80,000 barrels per day — more than the estimated volume of the spill.
The containment cap is a temporary solution to the broken well, which spewed oil into the Gulf for months following the April 20 rig explosion.