Human Rights Watch has published a report giving fresh details of alleged torture and abuse of detainees by US forces in Iraq.
US soldiers have been trying to gain intelligence from Iraqi prisoners
The report quotes three US soldiers who described routine, severe beatings of prisoners, including a detainee's leg being broken with a baseball bat.
Other allegations included applying burning chemicals to detainees' eyes and skin, making them glow in the dark.
A US defence spokesman said the report contained errors and distortions.
The Human Rights Watch (HRW) report is based on interviews with a captain and two sergeants who served in a battalion of the 82nd Airborne Division.
They said abuse, at a military base called Mercury near Falluja, was not only overlooked, but was sometimes ordered.
The punishments handed out included sleep deprivation, withholding food and water, "human pyramids" like those seen in photos from Abu Ghraib prison, and blows to the face, the report claimed.
One of the soldiers told HRW the abuse was ordered by intelligence officers in an attempt to gain information.
Another said it was seen as "sport".
"Everyone in camp knew if you wanted to work out your frustration you show up at the [interrogation] tent," he reportedly said.
"As long as no PUCs [prisoners under control] came up dead, it happened," he said.
"We kept it to broken arms and legs."
HRW said the reports "suggest that the mistreatment of prisoners by the US military is even more widespread than has been acknowledged to date".
Lt Col Skinner of the US Department of Defense said the dossier was trying to "advance an agenda through the use of distortions and errors in fact".
He said 400 investigations had been launched into prisoner abuse allegations and "looked at all aspects of detention operations under a microscope".