The scandal has also brought to light the growing and largely unregulated role of private contractors in the interrogation of detainees.

According to lawyers for some of the soldiers, they claimed to be acting in part under the instruction of mercenary interrogators hired by the Pentagon.

US military investigators discovered the photographs, which include images of a hooded prisoner with wires fixed to his body, and nude inmates piled in a human pyramid.

The pictures, which were obtained by an American TV network, also show a dog attacking a prisoner and other inmates being forced to simulate sex with each other. It is thought the abuses took place in November and December last year.

The pictures from Abu Ghraib prison have shocked the US army.

Brigadier General Mark Kimmitt, deputy director of operations for the US military in Iraq, expressed his embarrassment and regret for what had happened.

He told the CBS current affairs programme 60 Minutes II: "If we can't hold ourselves up as an example of how to treat people with dignity and respect, we can't ask that other nations do that to our soldiers."

Gen Kimmitt said the investigation began in January when an American soldier reported the abuse and turned over evidence that included photographs.

"That soldier said: 'There are some things going on here that I can't live with'."

The inquiry had centred on the 800th Brigade which is based in Uniondale, New York.

The US army confirmed that the general in charge of Abu Ghraib jail is facing disciplinary measures and that six low-ranking soldiers have been charged with abusing and sexually humiliating detainees.

Lawyers for the soldiers argue they are being made scapegoats for a rogue military prison system in which mercenaries give orders without legal accountability.

A military report into the Abu Ghraib case - parts of which were made available to the Guardian - makes it clear that private contractors were supervising interrogations in the prison, which was notorious for torture and executions under Saddam Hussein.

One civilian contractor was accused of raping a young, male prisoner but has not been charged because military law has no jurisdiction over him.

Hired guns from a wide array of private security firms are playing a central role in the US-led occupation of Iraq.

The killing of four private contractors in Falluja on March 31 led to the current siege of the city.

But this is the first time the privatisation of interrogation and intelligence-gathering has come to light.

The military investigation names two US contractors, CACI International Inc and the Titan Corporation, for their involvement in Abu Ghraib.

Titan, based in San Diego, describes itself as a "a leading provider of comprehensive information and communications products, solutions and services for national security".

CACI, which has headquarters in Virginia, claims on its website to "help America's intelligence community collect, analyse and share global in formation in the war on terrorism".

Neither responded to calls for comment yesterday.

According to the military report on Abu Ghraib, both played an important role at the prison.

At one point, the investigators say: "A CACI instructor was terminated because he allowed and/or instructed MPs who were not trained in interrogation techniques to facilitate interrogations by setting conditions which were neither authorised [nor] in accordance with applicable regulations/policy."

Colonel Jill Morgenthaler, speaking for central command, told the Guardian: "One contractor was originally included with six soldiers, accused for his treatment of the prisoners, but we had no jurisdiction over him. It was left up to the contractor on how to deal with him."

She did not specify the accusation facing the contractor, but according to several sources with detailed knowledge of the case, he raped an Iraqi inmate in his mid-teens.

Col Morgenthaler said the charges against the six soldiers included "indecent acts, for ordering detainees to publicly masturbate; maltreatment, for non-physical abuse, piling inmates into nude pyramids and taking pictures of them nude; battery, for shoving and stepping on detainees; dereliction of duty; and conspiracy to maltreat detainees".

One of the soldiers, Staff Sgt Chip Frederick is accused of posing in a photograph sitting on top of a detainee, committing an indecent act and with assault for striking detainees - and ordering detainees to strike each other.

He told CBS: "We had no support, no training whatsoever. And I kept asking my chain of command for certain things ... like rules and regulations."

His lawyer, Gary Myers, told the Guardian that Sgt Frederick had not had the opportunity to read the Geneva Conventions before being put on guard duty, a task he was not trained to perform.

Mr Myers said the role of the private contractors in Abu Ghraib are central to the case.

"We know that CACI and Titan corporations have provided interrogators and that they have in fact conducted interrogations on behalf of the US and have interacted the military police guards at the prison," he said.

"I think it creates a laissez faire environment that is completely inappropriate. If these individuals engaged in crimes against an Iraq national - who has jurisdiction over such a crime?"

"It's insanity," said Robert Baer, a former CIA agent, who has examined the case, and is concerned about the private contractors' free-ranging role. "These are rank amateurs and there is no legally binding law on these guys as far as I could tell. Why did they let them in the prison?"

The Pentagon had no comment yesterday on the role of contractors at Abu Ghraib, saying that an inquiry was still in progress.