A report from the US Government Accountability Office says tens of thousands of rifles, pistols and other items sent to Iraq are unaccounted for.
Pentagon officials are promising to keep closer track of weapons sent to Iraq.
The report does not say the missing weapons have gone astray, into the hands of insurgents or onto the open market.
Rather, it says that in its haste to provide the material to Iraqi forces in the midst of a war, the US military command in Iraq did not keep close enough track of rifles, pistols, body armor and helmets intended for the Iraqi army and police forces.
Pentagon Spokesman Bryan Whitman spoke about the report on Monday.
"One should not assume that just because you cannot provide a strict audit paper trail that those weapons are necessarily not being used for their intended purposes which is for the Iraqi security forces."
Whitman says he is not aware of any of the US weapons or other equipment being found in the possession of insurgents or in any of the thousands of insurgent weapons caches coalition and Iraqi forces have found around the country. But Whitman acknowledges, as the department does in a letter at the end of the GAO report, that it needs to improve its inventory system in Iraq.
"We acknowledge the fact that the systems that we have ought to be continuously improved upon and refined so that we can ensure that the Iraqi security forces have accountability systems that are effective and that provide for safeguarding equipment that is provided to their military and police forces."
The congressional investigators' report indicates that the US military command's records do not account for 110,000 AK-47 rifles, 80-thousand pistols, 135,000 pieces of body armor and 115,000 helmets. In all, it says that is about 30% of such equipment provided to Iraq.
Much of the discrepancy came during 2004 and 2005, when General David Petraeus was in charge of the effort to train and equip Iraq's new security forces. General Petraeus is now the top coalition commander in Iraq.
The report says has it not been clear what inventory system commanders in the field should use, and they have not had enough staff to monitor the large amount of equipment flowing in from the United States and out to Iraqi forces.
The Pentagon says it will take steps to address both problems.