BP's first successful effort to cap the ruptured oil well in the Gulf of Mexico held fast through Friday, but engineers will continue watching tensely for signs of more leaks in the coming hours and day.
While pressure readings were slightly below levels that authorities had hoped for, they remained "consistent" with a well that had not sprung any extra leaks, said Coast Guard Admiral Thad Allen, who is managing the government's response to the disaster.
Oil giant BP Plc on Thursday afternoon had temporarily stopped all oil flowing out of the ruptured well in the Gulf as it began its critical test of a new, tight-fitting cap placed on the leak earlier this week.
US President Barack Obama on Friday said the temporary stoppage was "good news", but cautioned that further tests were needed to check if the new cap would create fresh problems.
BP's "integrity test" marked the first time in nearly three months that the oil giant has managed to stop the oil flow. The oil spill brought on by the Deepwater Horizon rig explosion in April is already the worst in US history and has severely damaged the US Gulf Coast.
With the wellhead now shut, authorities said a pressure reading of 7,500 pounds per square inch (psi) or higher would be a clear sign that the well itself had remained intact.
Allen said the pressure stood at 6,700 psi on Friday afternoon and was still slowly rising. The reading marked a level of "ambiguity" that might indicate a leak or could simply mean that the reservoir was running out of oil.
Surface detection ships and robotic submarines operating near the spill sight had yet to detect any kind of leaks since the test began, Allen said. He also argued the "curve" in the pressure readings over the last 24 hours was "consistent" with a well that was still intact.
Allen suggested engineers had abandoned a 48-hour timeframe for conducting the test. They had shifted to a "conditions-based" approach and were re-evaluating every six hours, Allen said, meaning the test could go beyond 48 hours if needed.
Successful or not, the cap was a temporary solution. BP hopes a relief well that could permanently seal the well will be finished by mid-August. In the meantime, the cap could also be used to siphon oil to the surface instead of shutting the well completely.
"The new cap is good news as we'll either use it to stop the flow or use it capture all of the oil until the relief well is in place," Obama said.
But he cautioned that "we won't be done until we actually know that we've killed the well. We're making progress but we shouldn't get ahead of ourselves".
Obama said scientists were examining additional data to make sure that oil isn't seeping out elsewhere "in ways that could be more catastrophic".
In the almost 90 days that the disaster has unfolded, Obama has made four trips to the affected Gulf states, and said that he would go back "sometime in the next several weeks".
Up to 60,000 barrels of oil have been gushing daily from the ruptured well at the bottom of the Gulf since the Deepwater Horizon oil rig exploded April 20. BP's previous attempts to halt or siphon off the oil have had limited success.