The Red Cross discovered "serious violations" of the rights of Iraqi prisoners, with abuse so widespread it may be considered to have been tolerated by the US-led coalition.
In a confidential 24-page document, which was seen by the Wall Street Journal, the International Committee of the Red Cross said treatment in some cases was "tantamount to torture", particularly when interrogators were seeking information or confessions.
An official at the Geneva-based ICRC said the document, covering the period March-November 2003, was genuine, adding that the leak was a "major breakdown in confidentiality."
In a rare break with its normal practice, the ICRC said that it would release the full text.
The use of ill-treatment "went beyond exceptional cases and might be considered a practice tolerated" by coalition forces, the newspaper quoted the ICRC as saying.
That differs sharply from the view of senior officials in the administration of US President George W. Bush that military higher-ups had not condoned abuse, the newspaper said.
In the report, the ICRC said prisoners at Baghdad's Abu Ghraib prison were held in empty cells naked and beaten by soldiers.
The aid group also said coalition forces fired on unarmed prisoners from watchtowers and killed some of them, as well as committing "serious violations" of the Geneva Conventions governing treatment of war prisoners, the Journal said.
The newspaper report comes a day after the Red Cross said yesterday it had repeatedly urged the United States to take "corrective action" at a Baghdad jail at the centre of a scandal over abuse of Iraqi prisoners.
The Geneva-based humanitarian agency, mandated under international treaties to visit detainees, has had regular access to Abu Ghraib prison since US-led forces began using it last year, a Red Cross spokeswoman said.