Pentagon acknowledges hundreds of American service members alerted military medical staff they had been exposed to chemical weapons while serving in Iraq.
More than 600 US troops since 2003 reported that they were exposed to chemical agents in Iraq, a much higher number than the Pentagon stated previously.
The revelation was first reported by the New York Times, which has revealed in a series of articles this month that American troops handled an arsenal of deteriorating chemical agents and were sometimes told to stay silent about what they encountered.
The Pentagon failed to recognise the scale of the reported cases of exposure to chemical agents or to offer proper tracking and treatment to those troops who may have been injured, the Times wrote, citing defense officials.
Before the 2003 US invasion of Iraq, president George W. Bush insisted Baghdad was hiding an active weapons of mass destruction program.
US forces found no proof of an active program, but they did find remnants of an aging chemical stockpile that they were not well trained to handle.
The Times initially uncovered 17 cases of American troops who suffered damage from sarin or a sulfur mustard agent, and about eight more came forward.
But a new review of military records ordered by Pentagon chief Chuck Hagel has now found that hundreds of troops informed the military they believe they were exposed, officials said, confirming the Times report.
Hagel has ordered new medical exams for troops and veterans affected and officials said a national telephone hotline has been set up to allow people to report potential exposures and seek medical care.
Following the invasion that toppled Saddam Hussein, American forces uncovered 5,000 warheads, shells and bombs filled with chemical agents, but their findings were kept secret.