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Oil flows from one of three valves in the new cap atop the BP oil well in the Gulf of Mexico that has been leaking since it blew out April 20. This image was taken from video provided by BP on Thursday. ((BP/Associated Press))

Oil has stopped flowing from BP's broken wellhead into the Gulf of Mexico thanks to a new containment cap, the company said Thursday.

But officials with the company remained cautious as engineers began monitoring pressure gauges and watching for signs of leaks elsewhere in the well. 

The tests will continue in six-hour increments for up to 48 hours before the procedure will be considered successful.

Any new leaks would mean the cap would have to be reopened, which would cause oil to spill into the water.

"For the people living on the Gulf, I'm certainly not going to guess their emotions," BP vice-president Kent Wells said. "I hope they're encouraged there's no oil going into the Gulf of Mexico. But we have to be careful. Depending on what the test shows us, we may need to open this well back up."

Even if the well holds out for the whole two days of testing, the vents will be opened again and oil released while engineers conduct a seismic survey of the ocean floor to make sure oil and gas aren't breaking out of the well into the bedrock, said retired Coast Guard Adm. Thad Allen, the Obama administration's point man on the disaster.

"I think it is a positive sign," U.S. President Barack Obama said Thursday, referring to the stopped leak. "We're still in the testing phase. I'll have more to say about it tomorrow."

If pressure too great, oil to be diverted to surface

BP began testing the cap on Thursday afternoon, closing off openings in it one by one while monitoring pressure underneath.

The cap is intended to stop the oil from gushing into the sea by either holding all the oil inside the well machinery like a stopper or, if the pressure is too great, directing some through pipes to as many as four collection ships on the surface.

The new cap was lowered onto the well on Monday, but efforts to test the system have been repeatedly delayed.

Tests on it were halted on Wednesday after crews discovered oil leaking out of a pipe called a "choke line" that is attached to the sealing cap.

Allen told reporters Thursday morning that teams had replaced the choke line and would be checking it.

Allen said the periodic testing of the cap would also offer insight into the other, more permanent, solution to the fix: two relief wells intended to stop the gusher from deep underground by relieving pressure and diverting the oil flow.

'Break in the action'

The mapping of the sea floor that was done to prepare for the well cap test and the pressure readings will also help determine how much mud and cement will be needed to seal off the well.

Drill work was stopped on the relief well because it was not clear what effect the testing of the cap might have on it. Work on the other relief well had already been stopped according to plan.

The ultimate solution to stopping the blowout is to plug the wells, which can only be done when a relief well intersects with the old well bore.

Meanwhile, Allen said the prevailing winds are slowing the drift of oil to shore.

"We're getting a little break in the action as far as the oil closing [on] shore," he said. "It's given us a chance to kind of consolidate our forces and make sure we can redouble our efforts on onshore cleanup."

Since the oil well off the Louisiana coast blew out on April 20, setting off an explosion on the Deepwater Horizon oil rig, an estimated 689 million litres of oil have flowed into the Gulf.

Acy Cooper, vice-president of the Louisiana Shrimp Association, told CBC News that while his organization welcomed the news of the capped oil, its members are still preparing for tar balls to flow into their area from the millions of barrels already spilled into the Gulf. 

"It's gonna be an ongoing thing," he said. "Just because they stopped it, it doesn’t mean it's not coming. It's good that they stopped it, but that doesn't mean it's over. We know that."