The US government has tried to hide the extent to which its troops were exposed to chemical weapons in the aftermath of the 2003 invasion of Iraq, the New York Times reported Wednesday, citing intelligence documents and former soldiers.

American forces uncovered 5,000 warheads, shells and bombs filled with chemical agents but their findings were kept secret, the Times wrote, citing government papers obtained through a Freedom of Information Act request.

Before the 2003 invasion, president George W. Bush insisted Baghdad was hiding an active weapons of mass destruction program.

Although US forces never found evidence of an active program, they did find remnants of an aging chemical arsenal and often they were not trained nor equipped to handle it, according to the report.

Members of Congress were only partially informed about the chemical weapons and American soldiers were told to keep silent or to offer misleading accounts of what they found, the paper reported.

One former US Army sergeant who suffered from mustard burns in 2007 and was reportedly denied hospital treatment and medical evacuation told the Times: "I felt more like a guinea pig than a wounded soldier."

Responding to the article, the Pentagon said it has always been open about US forces finding chemical agents in Iraq as far back as 2006.

Officials did not confirm allegations that troops were instructed to play down injuries or hide their findings, but spokesman Rear Admiral John Kirby acknowledged that about 20 service members from 2004 to 2011 were exposed to nerve or mustard agents

"Right now our best estimate is, it's around 20 that we believe through that period ... were exposed to chemical ammunition," Kirby told a news conference.

An official tally had remained classified, according to the Times.

Defense Secretary Chuck Hagel "is concerned by any indication or allegation that our troops have not received the care and administrative support they deserve," Kirby said earlier in an email.

All of the chemical weapons found by US forces were made before 1991 in a rush by Saddam Hussein's regime to bolster its arsenal during its war with neighboring Iran, the paper said.

In most cases, the weapons were designed in the United States, manufactured in European countries and assembled in chemical agent production lines built in Iraq by Western firms, according to the Times.

Not only did government secrecy prevent some troops from receiving proper medical care for injuries from chemical agents, it also meant the soldiers lost out on receiving medals or other official recognition of their wounds, the paper reported.