HOUSTON (Dow Jones)--BP PLC'S (BP, BP.LN) attempt to kill the well that spewed millions of gallons of oil in the Gulf of Mexico will be delayed until early next week, as the company cleans up rock debris found at the bottom of a relief well, a spokesman said Friday. The start of the so-called "static kill," previously scheduled for late Sunday or early Monday, is now estimated to start on Monday or Tuesday, BP spokesman Steve Rinehart said. The reason for the delay is that rocks fell into the depths of a relief well that's bound to intersect the damaged Macondo well, and "it's taking some time to clean that out" before casing is inserted, Rinehart said. U.S. authorities say that casing is necessary to reinforce the relief well before BP can flood the neighboring damaged well with drilling mud to permanently kill the spill. Both wells are separated by a 4.5-foot wall of rock. Once the Macondo well is flooded from the top with mud and cement, BP will attempt to flood it from the bottom via the relief well. In a teleconference call on Friday, the head of the federal oil spill response effort, retired Coast Guard Admiral Thad Allen, said that cleaning the sedimentary debris out of the relief well "is not a huge problem." The debris fell after a drilling rig had to quickly evacuate the well last week as tropical storm Bonnie approached the area without being able to complete the casing of the well. Casing prevents rocks and other sediments from falling into the well. BP said Friday that oil remained shut in at the damaged well, and pressure kept building slowly, an indication that the well is in good shape and hydrocarbons are not seeping out from the sides. -By Angel Gonzalez, Dow Jones Newswires; 713-547-9214;angel.gonzalez@dowjones.com (END) Dow Jones Newswires July 30, 2010 14:20 ET (18:20 GMT)