The United States has spent over $51 billion to help Afghanistan field, clothe, arm and house a national security force, but the special inspector general for Afghanistan reconstruction Monday said that Washington doesn't even know the size of the Afghan force it's paying for.

And in a shocking statement, the IG, John Sopko, said that the numbers the U.S. is relying on from Afghan officials could be a sham, resulting in billions in waste.

Sopko, whose office is in charge of auditing the near $100 billion taxpayers are funnelling to rebuilding Afghanistan, said, "It looks like our data on the forces, the Afghan National Security Forces, that we are going to be relying on, may be bogus. We don't know what supports it."

His comments at a seminar hosted Monday by Center for Strategic and International Studies heightened concerns in his latest quarterly report on U.S. spending that taxpayers might be getting ripped off.

The report said that the Afghan security force totals 331,592, about 50,000 short of the goal of the combined army and police force. But then it warned that there is "no viable method of validating...personnel numbers."

The blunt-talking Sopko raised other concerns about how U.S. funds are being spent in Afghanistan. For example he noted that the Afghan government doesn't always want facilities the U.S. builds, leaving them nearly empty or unused. And he raised flags over plans by U.S. agencies to spend billions more before U.S. forces leave in late 2014. That, he said, could "waste billions" of dollars.

Sopko said that as troops leave, the areas his auditors will be able to safely go will be limited. His office is already looking to supplement personal audit visits with satellite viewing, but he conceded that the "gold standard" of auditing is "kicking the tires."

Without hands-on audit, he said, "I fear that many of our programs will exposed to increased risk of theft and misuse.