The U.S. has given Pakistan nearly $6 billion to pursue terrorists since the Sept. 11 attacks, but with little to no proof that the money has been used for that purpose, an independent audit has found.

The assessment by the Government Accountability Office, released Tuesday, angered members of Congress who say they are concerned that Pakistan -- the nation's closest ally in the war on terrorism -- is milking the U.S. government.

GAO found, for example, that the U.S. recently gave Pakistan more than $200 million for air defense radars without bothering to investigate whether the money was needed specifically to go after terrorists. Another example included $45 million for road and bunker construction without evidence they were ever built.

"The U.S. government is being asked to reimburse Pakistan for non-incremental air defense radar maintenance when al-Qaida is not even known to have an air force," said Rep. Howard Berman, D-Calif., chairman of the House Foreign Affairs Committee. "The purpose of these funds is to support the fight against extremists, not to boost Pakistan's conventional warfare capability."

Since the 2001 terrorist attacks, the U.S. has given Pakistan some $10.8 billion in aid. About half of the money, $5.6 billion, has been used to reimburse Pakistan for military operations. The country is by far the largest recipient of U.S. military aid, designated "coalition support funds," because of the influx of al-Qaida and Taliban fighters into its unpoliced tribal areas along the Afghan border.

"We need better oversight and visibility regarding where these funds are going," said Rep. Christopher Shays, R-Conn., the top Republican on the House Oversight and Government Reform's national security and foreign affairs subcommittee.

The Defense Department said the GAO report did not give sufficient weight to Pakistan's contributions to the war on terrorism, which officials say have been substantial.

"They've captured or killed more al-Qaida and Taliban than any other coalition partner and have sustained more than 1,400 combat deaths," Ret. Maj. Gen. Bobby Wilkes, deputy assistant defense secretary for South Asia, told the House subcommittee.

Wilkes added that "there are no easy answers" but said the program was among the most successful efforts available to fight terrorism.

GAO agreed that the program was critical, but concluded that the Defense Department still had not followed existing guidance to account for the money.

"As a result, we conclude that Defense cannot accurately determine how much of the ($5.6 billion) in costs reimbursed to Pakistan since 2001 were actually incurred," GAO wrote.

In a separate April report, GAO concluded that Pakistan had been unable to defeat terrorists inside its borders despite the influx of U.S. cash and that its security forces were not structured to target an insurgency and face equipment and training deficiencies.