Engineers monitored a newly-capped oil well in the Gulf of Mexico Sunday amid cautious optimism the months-long spill behind the worst environmental disaster in US history has been finally contained.
Still image from a live BP video feed shows no apparent oil leakage July 16 in the Gulf of Mexico.
The tests, which involve multiple pressure readings on the wellbore that runs to the oil reservoir below the seabed, have provided "valuable information" and will continue into 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 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 Kent Wells told reporters.
The tests began 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."
BP also brought in a ship from the National Oceanic and Atmospheric Administration to survey the area.
If there was a leak, BP would have to open the valves holding back the oil, and allow the crude to once again flow freely into the Gulf, experts said.
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.
Progress continues on two relief wells, expected to be completed in mid-August, which Allen recalled are "the ultimate step in stopping the BP oil leak for good."
Company officials said they had come within five feet (1.5 meters) of the original well casing on Saturday, and that the kill procedure could begin as early as August.
"We're feeling very good at this point about how the well is lining up," Wells said.
However, The New York Times reported late Saturday that with the encouraging news from the test, there had been discussions about leaving the well shut and changing the method of permanently sealing it.
Citing an unnamed technician involved in the discussions, the newspaper said engineers had discussed stopping work on the relief well in favor of a "bullhead kill."
In that operation, heavy mud would be pumped in through existing pipes and the oil and gas would be forced back into the oil reservoir at the bottom of the well, the report said.
Nothing but a white cap and underwater robots appeared Saturday on a video feed that had for weeks shown clouds of oil gushing forth at an estimated rate of 35,000-60,000 barrels a day.
However temporary, the halt provided a glimmer of hope that the worst oil spill in US history could soon be over, allowing efforts to turn to the grim job of cleaning up hundreds of miles (kilometers) of contaminated shorelines.
Oil has washed up on the coasts of all five Gulf states -- Texas, Louisiana, Mississippi, Alabama and Florida -- since the spill began on April 22, two days after an explosion ripped through the BP-leased Deepwater Horizon platform, killing 11 workers and sinking the rig.
The International Energy Agency estimates that between 2.3 million and 4.5 million barrels of crude are now sloshing around in the sea.
A giant skimming vessel, the "A Whale," officials had hoped would scoop up huge quantities of spilled crude was only able to collect negligible amounts during testing and will not be deployed.
Gulf residents, who depend heavily on the fishing and tourism industries have seen their livelihoods ravaged, and the complicated and expensive cleanup process is likely to take years.
BP has so far spent at least 3.5 billion dollars dealing with the spill, and compensation claims could eventually cost 10 times that amount. BP has already agreed to set up a 20-billion-dollar fund to pay damages.