The pictures, which have not been released to the public by the US military, were described by one member of Congress as worse than had been expected.

"I expected that these pictures would be very hard on the stomach lining and it was significantly worse than anything that I had anticipated," Senator Ron Wyden told reporters. "Take the worst case and multiply it several times over."

The pictures are thought to depict scenes of torture and humiliation similar to those seen in the photographs that have emerged over the last week. Photographs of dogs snarling at prisoners, of women being forced at gunpoint to expose their breasts, of hooded prisoners being forced to masturbate, and of forced homosexual acts were among those shown to members of Congress yesterday.

"The whole thing is disgusting and it's hard to believe that this actually is taking place in a military facility," said Senator Dianne Feinstein.

The lawmakers were shown the photographs by Defence Department officials in a secret room in the Capitol building in Washington DC.

Congresswoman Ellen Tauscher, a member of the House of Representatives armed services committee who viewed some of the pictures, told the Guardian they were "not dramatically different" from those already published but said some of the pictures showed the aftermath of a dog attack on a prisoner.

While several Republican senators argued that the photographs were no more shocking than those already seen, Democrats such as Senator Joseph Lieberman argued that "it just deepens the conclusion that this was a cellblock that had gone wild, had no standards".

The sense of disgust was compounded yesterday by the airing of an American soldier's video diary on CBS's 60 Minutes. The video showed her talking about two Iraqi prisoners who died in custody at Abu Ghraib prison: "Who cares? That's two less for me to worry about."

In evidence to a Senate committee yesterday, Mr Rumsfeld defined prisoners in Iraq as "unlawful combatants" rather than prisoners of war but insisted they were treated in a manner "consistent with" the Geneva conventions.

Further down the chain of command, it was announced that two US army sergeants would face court martial for their part in the abuses at Abu Ghraib. Staff Sergeant Ivan "Chip" Frederick and Sergeant Javal Davies join four other military police personnel who have already been charged.

One of those charged, Private Lynndie England, who featured prominently in the first batch of photographs, yesterday insisted she was acting on orders from "persons in my chain of command".

"I was instructed by persons in higher rank to 'stand there, hold this leash, look at the camera', and they took pictures for PsyOps [psychological operations]," Pte England told a Denver television station. "I didn't really ... want to be in any pictures."

The controversy over the treatment of prisoners by the US army spread to Afghanistan when an Afghan police colonel, Sayed Nabi Siddiqui told reporters from the New York Times and Associated Press he had been repeatedly beaten, stripped naked and threatened with dogs for nearly 40 days last year at several US-run bases in Afghanistan.