Rumsfeld says sorry but warns of more photos
Mr Rumsfeld took full responsibility for the "grievous and brutal abuse and cruelty", saying it happened "on my watch".
But he brushed aside Democratic demands for his resignation. Asked if he could remain effective in his post, he said if he believed he could not, "I'd resign in a minute".
He promised compensation for the victims but warned there were more pictures of "blatantly sadistic" treatment of prisoners to come.
He announced a review of his defence department and the appointment of a team of former top officials to investigate the ongoing inquiry into the abuses.
Giving evidence to a congressional committee in Washington, Mr Rumsfeld also apologised for not keeping the US president and Congress informed.
"These events occurred on my watch. As secretary of defence I am accountable for them and I take full responsibility.
"I feel terrible about what happened to these Iraqi detainees. They are human beings, they were in US custody, our country had an obligation to treat them right. We didn't and that was wrong.
"To those Iraqis who were mistreated I offer my deepest apology. It was inconsistent with the values of our nation. And it was fundamentally un-American," he said.
During the session, which was interrupted by protesters shouting "fire Rumsfeld", the defence secretary said the abuses were the work of a small number of soldiers and the vast majority of troops were "truly wonderful human beings".
He said he deeply regretted the damage the scandal had done to their reputation. But he warned: "There are other photos that depict incidents of physical violence to prisoners, acts that can only be described as blatantly sadistic, cruel and inhuman.
"There are many more photographs and indeed some videos. If these are released to the public, obviously it's going to make matters worse," he said.
Thus far, no videos of abusive treatment have reached the public. However, the stills have spawned a global wave of revulsion that has damaged America's image overseas.
Meanwhile, in Geneva yesterday, the International Red Cross said it warned US officials of abuse of prisoners in Iraq more than a year ago.
"We were dealing here with a broad pattern, not individual acts," said Pierre Kraehenbuehl, director of operations for the International Committee of the Red Cross.