WASHINGTON (CNN) -- The United States has not accurately tracked about $6 billion it gave to help the Pakistani government fight terrorism since 2001, according to a report released Tuesday.

The Department of Defense says Pakistani troops have been key allies in the war on terrorism.

Pakistan is the largest recipient of payments from the Coalition Support Funds, which gives money to 27 partner countries to help combat terrorism.

The country, which the Department of Defense considers a key ally in the war on terrorism because of its proximity to large swaths of ungoverned tribal land, has received $5.56 billion of $6.88 billion given out since September 11, 2001.

But a report released Tuesday by the Government Accountability Office and the House Committee on Foreign Affairs said the Coalition Support Funds cannot prove that the money went to projects or operations specifically fighting terrorists.

While calling the program critical to the war on terrorism, the GAO found numerous concerns about the accuracy of the Pakistani accounting.

"As a result, we conclude that Defense cannot accurately determine how much of the $5.56 billion in costs reimbursed to Pakistan since 2001 were actually incurred," the report said.

The report drew ire from Congress, as some members said Pakistan is bilking the United States to "boost its conventional warfare capability," committee Chairman Howard L. Berman said.

The Department of Defense responded to the report by saying Pakistan has contributed significantly to the war on terrorism but acknowledged some problems tracking the money.

"At the U.S. request, the Pakistan Army deployed major ground forces for the first time to the Federally Administered Tribal Areas to attempt to capture the remnants of al Qaeda and Afghan Taliban fleeing into Pakistan from the Tora Bora Area of Afghanistan," the department said.

"The cost to Pakistan of these operations, in human terms, has been significant. Approximately 1,400 Pakistani security forces members have lost their lives since 2001 in the [Global War on Terrorism]," the statement continued.

The watchdog arm of Congress found the Defense Department paid more than $200 million for an air defense radar but failed to check why it was needed, because the Taliban and al Qaeda do not have air forces.

The report also cited payments of about $45 million to the Pakistani government for road and bunker construction for the Army. The United States failed to get evidence that the roads and bunkers were ever built.

At times, the United States may have paid more than once for the same thing. The GAO found that Pakistan received about $3.7 million each year for to operate a fleet of fewer than 20 vehicles for the country's Navy.

The GAO said the United States paid for "vehicle damage" and "cost of vehicles repaired" without any explanation of the difference between the two.

Defense Department officials have said Pakistan is the United States' key ally in the war on terrorism because of the country's location.

The vast, ungoverned tribal area known as the Federally Administered Tribal Area is considered a protected region for terrorists. The area is in western Pakistan, along the Afghan border.

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