Once a month, Pakistan's Defense Ministry delivers 15 to 20 pages of spreadsheets to the U.S. Embassy in Islamabad. They list costs for feeding, clothing, billeting and maintaining 80,000 to 100,000 Pakistani troops in the volatile tribal area along the Afghan border. No receipts are attached. In response, the Defense Department has disbursed about $80 million monthly, or roughly $1 billion a year for the past six years, in one of the most generous U.S. military support programs worldwide. But vague accounting, disputed expenses and suspicions about overbilling have recently made these payments to Pakistan highly controversial -- even within the U.S. government. Questions have already been raised about where the money went and what the Bush administration got in return. In perhaps the most disputed series of payments, Pakistan received about $80 million a month in 2006 and 2007 for military operations during cease-fires with pro-Taliban tribal elders along the border, including a 10-month truce in which troops returned to their barracks. U.S. officials say the payments to Pakistan -- which over the past six years have totaled $5.7 billion -- were cheap compared with expenditures on Iraq, where the United States now spends at least $1 billion a week on military operations alone. Congressional officials and others are concerned that the administration has been so eager to prop up Musharraf that it overlooked U.S. foreign aid and accounting standards. A congressional oversight subcommittee is also set to begin an investigation next month, while the Government Accountability Office plans to finish its own inquiry in April.