BP gingerly moved ahead with tests of its newly sealed oil well Friday, stepping up scanning the sea floor for leaks that would signal potential problems with an operation that has rarely gone as planned.

The gusher of oil that has been fouling the gulf since late April was turned off Thursday when BP closed a valve on a new well cap, erasing the roiling clouds of crude that day after day have fed the largest oil spill in U.S. history.

But the underwater video scenes of clear water may just be temporary. If the pressure in the sealed wellhead doesn't rise to a certain point and stay steady, that will indicate oil is seeping into the seabed from the well hole, a situation that could make matters worse.

If that happens, BP will immediately open the cap valves, let oil escape from the top and resume a collection operation that has been capturing some of the leak and funneling it to ships.

A key test designed to reveal how intact – or damaged – the deep-sea well hole is got off to a good start early Friday when well pressure readings rose to 6,700 pounds per square inch. But when the pressure increase stalled, federal officials ordered BP to increase monitoring.

"We're at the point where there's enough uncertainty … we need to be careful not to do any harm," said Thad Allen, the retired Coast Guard admiral who is overseeing the spill response.

After consulting with scientists Friday afternoon, Allen told BP to take more seismic soundings of the seabed and expand its monitoring of the seafloor. A federal ship with the ability to detect methane bubbles in the water – signaling a leak – was also called into action.

Allen said there are alternate explanations of why the pressure rise had not been greater once the well was "shut in" by the cap: There could be a breach in the well hole, or three months of leaking could have disgorged so much crude that the underlying oil reservoir is depleted.

"We're taking this one step at a step," Allen said, adding that engineering teams are reevaluating the test procedure every six hours.

-- Bettina Boxall