US Defence Secretary Donald Rumsfeld has offered his "deepest apologies" to Iraqi inmates mistreated by US troops.
Rumsfeld joined the president in deploring the Iraqi prisoners' humiliation (AP/Courtesy The New Yorker)
He told Congress he bore responsibility for events that had taken place "on his watch", and felt terrible.
Asked if he would step down to remedy the damage done to America's reputation, Mr Rumsfeld said he would not do so under political pressure.
He also revealed that he expected more photographs and a videotape of alleged prisoner abuse to emerge.
The scandal developed after pictures were published last week, showing prisoners being mistreated at the Abu Ghraib prison in Baghdad.
In separate testimonies before the Senate and later the House Armed Services Committee, Mr Rumsfeld called such acts "blatantly sadistic, cruel and inhuman".
There were also "many more photographs and indeed some videos" to come, he warned.
Mr Rumsfeld recognised that his department had been slow to notify Congress about the allegations, but denied that there had been any attempt at a cover-up.
He said reports of abuse had been properly investigated and reported by the military.
He added that he had not realised the seriousness of the allegations until pictures were leaked to journalists.
Mr Rumsfeld told senators: "I failed to recognise how important it was to
elevate a matter of such gravity to the highest levels, including to
the president and members of Congress."
"I wish I had been able to convey to them the gravity of this before we saw it in the media."
US military justice, Mr Rumsfeld said, deals with 18,000 criminal investigations every year.
These take time and there is no procedure to alert senior commanders to damaging cases that need to be treated urgently, he told both committees.
Remedying this while continuing to protect defendants' rights was a challenge, he said. "We're going to figure out a way to do it."
Mr Rumsfeld - who appeared flanked by senior defence officials, including the chairman of the US Joint Chiefs of Staff, Gen Richard Myers - also said he had created a special commission to investigate the abuse.
Faced by calls to step down he said: "Needless to say, if I thought I could not be effective, I would resign in a minute."
He added that he would not resign "simply because people try to make a political issue out of it".
Shouting members of the public interrupted Mr Rumsfeld's opening statement at the Senate, demanding that Mr Rumsfeld be sacked and that the US investigate a broad range of alleged human rights abuses in Iraq.
The BBC's Justin Webb in Washington says Mr Rumsfeld's abrasive style has won him many enemies on Capitol Hill.
The apology was the easy bit and Mr Rumsfeld remains on the defensive, our correspondent says.
Reaction among lawmakers has been mixed. Some Democratic senators - including Edward Kennedy - called for his dismissal.
A number of Republicans are also said to be unhappy with the defence secretary.
But there was strong backing from John Warner, the powerful chairman of the Senate Armed Services Committee.
He said he intended to support the decision by President Bush to keep Mr Rumsfeld on.