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NEW YORK (Dow Jones)--Testing of BP PLC's (BP, BP.LN) new containment cap that has cut off the flow of oil from a leaking underwater well in the Gulf of Mexico will continue for another 24 hours, U.S. Coast Guard Admiral Thad Allen said Saturday in a statement. The test was originally scheduled to conclude Saturday afternoon, but is being extended to allow for additional monitoring of the surrounding sea floor for signs of new leaks created by the buildup in pressure since the cap was installed. Pressure in the well has climbed more slowly than expected over the two days the cap has been in place, though BP and government testing have found "no evidence" that the integrity of the seabed is weakening, BP Senior Vice President Kent Wells said earlier Saturday. The cap, which has stopped oil from leaking, is undergoing at least two days of testing. Pressure has reached 6,745 pounds per square inch and is expected to max out at 6,800 psi, Wells said in a media briefing. That's less than the 7,500 psi initially hoped for, but so far does not indicate that the device is causing oil to escape elsewhere. A government sonar ship is being brought in to monitor the seabed, in addition to remotely operated vehicles patrolling the area. "The longer the test goes, the more confidence we have in it," Wells said. The tests are meant to quell fears that the containment cap will cause a rupture elsewhere on the seabed, creating new leaks. The capped well was estimated to be gushing between 35,000 and 60,000 barrels of oil a day into the Gulf, and has leaked ever since the Deepwater Horizon drilling rig caught fire and sank in April. Meanwhile, BP completed another "ranging run" in the furthest along of its two relief wells that aim to cut off the leak nearly 18,000 feet below the surface. The test, designed to pinpoint the location of the broken well, is one of the final steps BP will take before drilling the final 24 feet to the relief well's destination. Wells said drilling should wrap up by the end of July, while "killing" the leaking well by pumping in heavy fluid could take "days up to a few weeks." A relief well remains the only way to permanently stop the flow of oil, BP has said. "We're feeling very good at this point on how the well is lining up," Wells said. Drilling of a second relief well was suspended to avoid interfering with the first well, BP said. -By Brian Baskin, Dow Jones Newswires; brian.baskin@dowjones.com (END) Dow Jones Newswires July 17, 2010 17:37 ET (21:37 GMT)