Since 2004 the military "has not consistently collected supporting records confirming the dates the equipment was received, the quantities of equipment delivered, or the Iraqi units receiving the items", the report said.
The US government cannot account for 190 000 weapons issued to Iraqi security forces in 2004 and 2005, according to an investigation carried out by the Government Accountability Office.
According to the July 31 report, the military "cannot fully account for about 110 000 AK-47 assault rifles, 80 000 pistols, 135 000 items of body armour and 115 000 helmets reported as issued to Iraqi forces".
The weapons disappeared from records between June 2004 and September 2005, as the military struggled to rebuild the disbanded Iraqi forces from scratch amid mounting attacks from Sunni insurgents and Shi'ite militias.
Since 2004 the military "has not consistently collected supporting records confirming the dates the equipment was received, the quantities of equipment delivered, or the Iraqi units receiving the items", the report said.
"Since 2006 the command has placed greater emphasis on collecting the supporting documents. However, GAO's review of the January 2007 property books found continuing problems with missing and incomplete records."
US commanders often accuse foreign powers such as Iran of supplying arms to illegal militias fighting in Iraq, but the report shows they cannot fully account for the hundreds thousands of weapons they brought in themselves.
Last month, Turkey raised concerns over reports that separatist Kurdish guerrillas launching cross-border raids from northern Iraq had received US-supplied guns supposedly destined for Iraqi security forces.
Billions spent on Iraqi security forces
The United States has spent .2bn on Iraq's security forces since the 2003 invasion to oust Saddam Hussein, with .8bn devoted to equipping them.
The report comes as US President George W Bush is under intense pressure from a Democrat-led Congress and critics within his own Republican party to show progress on Iraq, with many in both parties calling for withdrawal.
But the ability of Iraqi forces to stabilise the country in the wake of a US troop drawdown has been called into question, most recently by a mid-July progress report issued by the White House.
Prime Minister Nuri al-Maliki's government "has made unsatisfactory progress toward increasing the number of Iraqi security forces units capable of operating independently", that report said.
The report also found "no momentum in the government of Iraq toward developing and implementing a comprehensive disarmament programme for militia members" from Iraq's divided communities.
Four years after the 2003 US-led invasion the country remains in the grip of several overlapping conflicts, and Iraqi security forces, particularly the police, are widely believed to be infiltrated by rival militias.
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Original: 190 000 weapons missing in Iraq