Engineers probed BP's ruptured Gulf oil well for leaks on Saturday but said a giant cap placed on it two days ago appears to be holding despite puzzling pressure readings.
BP insisted after the first 24 hours of tests that the cap was holding after being shut on Thursday and that the wellhead was withstanding the force of oil bubbling up from the reservoir deep under the Gulf of Mexico.
"We haven't seen any indication... that we have got any oil or gas escaping from the well," BP senior vice president Kent Wells told reporters after the oil flow was choked off for the first time in three months.
BP's optimism came after analyzing temperature readings, data from acoustic sensors listening for any flow of oil and scanning pictures from robotic cameras working a mile down (1,600 meters) on the sea floor.
Tests are due to last until late Saturday, after BP engineers on Thursday closed three valves on the new cap on top of the fractured wellhead stopping the flow of oil.
US President Barack Obama said the halt to the oil flow is "good news," but cautioned: "It is important that we don't get ahead of ourselves."
He acknowledged there is still "an enormous amount of work to do," but called on Americans to remain positive.
Government pointman, Coast Guard Admiral Thad Allen, said although the results are "generally good news," the critical pressure readings are not as high as had been expected at the halfway stage of the 48-hour tests, and it is unclear why.
"We need to be careful not to do any harm, or create a situation that cannot be reversed," he cautioned.
Allen said the administration has ordered BP to carry out even more seismic and acoustic surveys, due to the inconclusive pressure readings.
High readings mean the well is still intact and there is no seepage into the sea floor. Low readings would indicate the casing of the wellbore was damaged in the April explosion and oil is escaping.
The pressure readings after 24 hours were middling at about 6,700 psi (461 bar), which Allen said meant "there was probably some ambiguity regarding the flow."
Two possible scenarios had emerged. "One of them could be the pressure is lower because we have depleted the reservoir," Allen said.
The second possibility is that the oil was leaking "someplace in the well itself," Allen said. "We don't know, as we don't know the exact condition of the wellbore."
The cap's apparent success is a glimmer of hope that the worst US oil spill in history may soon be capped, allowing efforts to turn to the grim job of cleaning up hundreds of miles (kilometers) of contaminated shoreline.
"I was jumping up and down for a while when I saw it was capped," said O'Neil Sevin, who runs a bayou-side shop selling bait, seafood, beer, tackle and snacks to recreational fishermen in Chauvin, Louisiana.
Estimates suggest anywhere between 35,000 and 60,000 barrels of crude have been leaking into the Gulf every day since the rig sank April 22, two days after an explosion on Deepwater Horizon platform which killed 11 workers.
The International Energy agency has calculated there are between 2.3 million and 4.5 million barrels of crude sloshing around in the sea.
A permanent end to the spill is not expected before mid-August, when two relief wells should enable BP to fill the ruptured wellbore with cement, drowning the oil flowing up from a huge undersea reservoir.
The wellbore stretches for several miles below the floor of the sea and any underground damage could trigger new, possibly catastrophic leaks.
Gulf residents, who are heavily dependent on the fishing and tourism industries, have seen their livelihoods ravaged and the complicated and expensive clean-up process is likely to take years.
Endangered wildlife has also been increasingly threatened by huge ribbons of oil fouling the shores of Texas, Louisiana, Mississippi, Alabama and Florida.
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, with BP agreeing to set up a 20-billion-dollar fund to pay damages.