HOUSTON (Dow Jones)--BP PLC (BP, BP.LN) reported Tuesday a slight increase in the pressure readings on a recently-installed containment device that's shutting in a massive oil spill in the U.S. Gulf of Mexico. The readings, posted on BP's website Tuesday, indicate that pressure, currently at 6,825 pounds per square inch, has risen slowly from 6,811 on Monday. But it's still far less than engineers' original expectations of 8,000 to 9,000 psi. Although BP says that the moderately-rising pressure is a sign that the oil reservoir is somewhat exhausted, U.S. authorities have expressed concern that it might indicate lateral seepage from the well. U.S. officials say that if any damage is detected, they'll order the company to restart the flow of the well. The well, which once flowed at a rate of up to 60,000 barrels of oil a day, has been shut in since Thursday, when BP began testing a new containment cap. BP declined to comment its latest web site update. BP canceled its regularly scheduled morning technical briefing Tuesday. BP plans to have a briefing later Tuesday but was unsure at what time, a company spokeswoman said. Retired U.S. Coast Guard Adm. Thad Allen, head of the federal oil spill response effort, has postponed his daily update. His office has not yet said when he plans to speak. On Monday, Allen gave BP permission to continue testing the cap until Tuesday afternoon. The cap was originally supposed to be tested for 48 hours but Allen has granted three 24 hour extensions. Allen holds the power to call off the test of the cap at anytime. -By Susan Daker, Dow Jones Newswires; (713) 547-9208; susan.daker@dowjones.com (END) Dow Jones Newswires July 20, 2010 13:07 ET (17:07 GMT)