The Swiss-run International Committee of the Red Cross (ICRC) says it warned Washington a year ago about alleged Iraqi prisoner abuse.
The ICRC said in a leaked report it had discovered widespread abuse and "serious violations" of the rights of inmates inside Iraq's Abu Ghraib prison.
"Our findings were discussed at different moments between March and November 2003, either in direct face-to-face conversations or in written interventions," Pierre Kraehenbuehl, the ICRC's director of operations, told a press conference on Friday.
Kraehenbuehl declined to give details of the contents, but confirmed that a leaked ICRC report to US authorities, published by the "Wall Street Journal", was genuine.
According to the newspaper, the ICRC document described the treatment of some prisoners as "tantamount to torture".
Abuses ranging from prisoners being kept naked in dark, empty cells to being shot dead by soldiers, it continued.
The report was sent to Washington in February and represented a summary of the information given to US authorities during the previous year, Kraehenbuehl said.
It comes a day after the ICRC said it had repeatedly urged the US to take "corrective action" at the Baghdad jail.
US Defence Secretary Donald Rumsfeld is facing mounting pressure to resign over the handling of the scandal. He offered Congress his "deepest apology" on Friday.
ICRC chief spokeswoman, Antonella Notari, told swissinfo that ICRC did not publicise the allegations of abuse when they first emerged because this would have compromised the humanitarian organisation's neutrality.
“The ICRC is active in over 70 countries worldwide and we visit more than 460,000 detainees. If we were to make public what we discuss with prisoners and what we see in prisons, we would no longer be granted access [to them]," Notari said.
"And that is important to us – to be present and to be able to act on the spot."
The ICRC's visits have taken place every five or six weeks since last year. The latest was on March 20.
“[We have] been granted unimpeded access to all detainees and all sections of the prison since it was used by US forces last year,” Notari said.
The scandal over treatment of prisoners began when a US television network broadcast pictures of smiling American guards with Iraqi prisoners in humiliating positions.
The images – as well as photographs published in the media around the world – sparked a huge international outcry.
But ICRC officials noted that the US had taken action on some of the issues but not on others. "There were situations that remained unacceptable and difficult and there were others that were worked on," Kraechenbuehl said.
The ICRC is designated by the Geneva Conventions to visit prisoners of war and other people detained by an occupying power.
It traditionally discusses its observations only with the detaining authority, but has been under pressure to say whether it had specifically warned the US about prisoner abuse before the photographs came to light.
The ICRC has broken its vow of silence on only a few occasions in the past.
The organisation recently expressed concerns about the conditions and treatment of Afghan and other detainees at the US naval base on Guantanamo Bay, Cuba.
swissinfo with agencies
The neutral, Swiss-run International Committee of the Red Cross (ICRC) was established in 1863 as the International Committee for Relief to the Wounded.
It has since helped millions of prisoners of war and victims of conflict by providing humanitarian aid, tracing missing persons and helping PoWs contact their families.
Switzerland is the depository of the Geneva Conventions, which outline rules concerning the treatment of prisoners of war.
The United States is the ICRC's largest financial contributor, donating around SFr200 million ($155 million) each year.