US Defence Secretary Donald Rumsfeld has promised that any Americans abusing Iraqi prisoners will be punished.
A soldier says he was ordered to photograph Iraqi detainees (AP/Courtesy The New Yorker)
The US military says there have been investigations into 25 deaths in US custody in Iraq and Afghanistan.
In two cases the dead men were found to have been murdered by Americans, according to a US army official.
Senior US politicians have called for public hearings on mistreatment of prisoners, and have demanded the right to question Mr Rumsfeld.
Angry senators said they had been kept in the dark by the defence department until photographs of apparent abuse emerged in the media.
But Mr Rumsfeld said armed forces chiefs acted swiftly and properly as soon as the claims came to light in January.
Mr Rumsfeld said those responsible for the "unacceptable and un-American" conduct would be brought to justice.
The Pentagon has confirmed that criminal charges have been filed against six US soldiers in relation to the photos, while six senior officers have been reprimanded.
But there have been concerns that the mistreatment is more widespread.
A senior army official said there had been investigations into 25 cases of death and 10 of abuse in US custody in Iraq or Afghanistan since December 2002.
The BBC's Pentagon correspondent Nick Childs says of the 25 deaths, 12 were found to be either of natural or "undetermined" causes, one was a "justifiable homicide", and two were murders. Ten inquiries are ongoing, he says.
An Army official said a soldier had been convicted of using excessive force when he shot dead a prisoner who was throwing stones at him.
He was thrown out of the army but did not go to jail.
The other murder was committed by a private contractor who worked for the CIA, the official said.
Following the emergence of the photos, taken at the Abu Ghraib prison outside Baghdad, army chiefs were called before an emergency hearing of the Senate Armed Services Committee.
Senators were angry that they had not been alerted that an investigation into abuse had taken place.
"The ramifications are so serious and so severe, and the implications are so grave, that that report should have been forthcoming here immediately," said the committee's top Democrat, Senator Carl Levin.
The internal report by Maj Gen Antonio Taguba found evidence of "sadistic, blatant, and wanton criminal abuses of Iraqi prisoners", including sexual abuse.
TAGUBA REPORT FINDINGS
Detainees threatened with loaded pistol
Inmates beaten and sexually abused
Prisoners photographed in sexual positions
Detainees threatened with dogs
The abuse of Iraqi detainees has been condemned across the US political spectrum including by President George W Bush.
But Mr Rumsfeld also defended the actions of the armed forces, saying they had acted promptly and properly, launching an investigation in January the day after abuse allegations were first made - and issuing a press release two days after that.
As the US tried to contain the damage caused as the pictures of abuse were shown in the press in the Arab world, US National Security Adviser Condoleezza Rice appeared on the Al-Jazeera channel to appeal for trust.
"The president guarantees that those who did that be held accountable... and people will see that we are determined to get to the truth," she said.
But in Iraq, the US-appointed human rights minister,
Abdul-Basat al-Turki, resigned on Tuesday in
protest at the abuses.
Meanwhile a lawyer for one of the soldiers allegedly involved in the abuse cases at Abu Ghraib said they were simply "following orders".
Guy Womack, attorney for Charles Graner Jr, said the campaign was coordinated by governmental agencies, including the CIA.
The former head of the prison, Brig Gen Janis Karpinski, said she believed military commanders were trying to shift the blame onto her and other reservists and away from the intelligence officers still at work in the prison.