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(Updates with new details) By Angel Gonzalez Of DOW JONES NEWSWIRES HOUSTON (Dow Jones)--The top U.S. government official overseeing the oil spill response effort said late Sunday said that a seep near the shut-in Macondo well in the U.S. Gulf of Mexico as well as other problems call for close monitoring of the seafloor while BP Plc (BP, BP.LN) keeps testing a new containment system. In a letter to BP Chief Managing Director Robert Dudley, retired U.S. Coast Admiral Thad Allen said that monitoring of the seabed is "of paramount importance" due to the seep and to "undetermined anomalies" at the wellhead. The letter asked BP to notify the government of any seepage within four hours of detection, and requested a written plan for rapidly resuming the flow from the well if hydrocarbons are seen leaking from the sea floor. The letter comes after BP said that the testing of a containment cap, which has been proceeding without problems and has shut-in the flow of oil from the Macondo well since Thursday, could continue until the mile-deep leak is plugged by a relief well in mid-August. Allen had said in an earlier statement that the test could be extended in incremental 24-hour periods if the company carefully monitored the seafloor. The U.S. government's science team must approve the extension, Allen said. Even though the continued shut-in of the well raises the possibility that one of the worst-uncontrolled oil gushers in the U.S. history has come to an end, Allen's latest statements show that the U.S. government has concerns about the stability of the new containment system devised by BP. In his letter to Dudley, Allen reiterated that the drilling of a relief well--which would permanently kill the spill by flooding the Macondo well with cement--should remain the foremost option to stop the gusher. The comment comes amid discussions that BP might use the new containment system to control the leak by flooding the well from the top. A BP spokesman said the company is reviewing the letter and is "continuing to work very closely with the government." The test, originally scheduled to last 48 hours, was extended on Saturday to allow for additional monitoring of the sea floor for signs of new leaks. But BP Chief Operating Officer Doug Suttles said in a teleconference Sunday morning that results so far have been encouraging. Pressure is building slowly, but the readings are lower than expected--a sign that the oil reservoir is partially depleted, Suttles said. However, Allen said in a Sunday statement that the lower-than-expected pressure readings could be a problem. Allen acknowledged the possibility of reservoir depletion, but also said that the readings could be explained by potential leakage caused by damage to the well. "While we are pleased that no oil is currently being released into the Gulf of Mexico and want to take all appropriate action to keep it that way, it is important that all decisions are driven by the science," said Allen. "Ultimately, we must ensure no irreversible damage is done which could cause uncontrolled leakage from numerous points on the sea floor." In an early morning conference call, Suttles said that BP is "hopeful that if the encouraging signs continue we'll be able to continue the integrity test all the way to the point we get the well killed," said Suttles. "Right now there is no target set to open the well back up to flow." Suttles added that reopening the well remains a possibility if problems are encountered, but so far there have been no issues. "We're just taking this day by day," he said. The first of two relief wells being drilled to permanently kill the leaking Macondo well by flooding it with cement could reach its target by late July, Suttles said. But the kill operation, which involves flooding the leaking well with cement, could last until mid-August, he added. BP will conduct a "ranging run" to determine the location of the first relief well, and will insert the last string of casing into the well, an operation that may take a week to complete, Suttles said. Then the company will resume drilling to finally intersect the Macondo well, a very slow process scheduled to begin in the last week of July. "All things are looking good at this point," Suttles said. BP still has to drill 100 feet vertically and four feet horizontally to reach its target, Suttles added. The capped well was estimated to be gushing between 35,000 and 60,000 barrels of oil a day into the Gulf, beginning shortly after the April 20 blast aboard Transocean Ltd.'s (RIG) Deepwater Horizon rig, which BP had leased to drill a well in its Macondo prospect. The spill, now one of the largest in U.S. history, has had a severe impact on fishing and tourism along the Gulf Coast. BP has paid more than $200 million in damage claims to residents and businesses, and has set up a $20 billion fund to pay for damages related to the spill. Suttles said that as of Sunday morning, the well's pressure was at 6,778 pounds per square inch, and rising at a rate of 1 or 2 psi per hour. Suttles reported the presence of bubbles in the area being monitored. He said that BP does not believe the bubbles are related to hydrocarbons, but they're being closely watched, he said. -By Angel Gonzalez, Dow Jones Newswires; angel.gonzalez@dowjones.com; 281-536-3064 (Tom Barkley, Brian Baskin and Ben Casselman contributed to this story) (END) Dow Jones Newswires July 18, 2010 21:38 ET (01:38 GMT)