The Pentagon will soon release "hundreds" of photographs showing alleged abuse by US personnel of "war on terror" detainees during the Bush administration, a US official said Friday.

The photos, including images from prisons in Iraq and Afghanistan, are being released in response to a long-running lawsuit and are sure to fuel a growing controversy over the treatment of terror suspects on former president George W. Bush's watch.

"I think it will be in the hundreds," said the Pentagon official, referring to the number of photos to be released for the first time.

The Defense Department confirmed it had agreed to release a "substantial" number of photographs by May 28 in response to a Freedom of Information Act lawsuit filed by the New York-based rights group, the American Civil Liberties Union (ACLU).

The photographs come from more than 60 criminal investigations from 2001-2006 of military personnel suspected of abusing detainees, said the defense official, who spoke on condition of anonymity.

The case dates back to a Freedom of Information request in 2003.

After a federal appeals court ruled against the government last year, administration officials had to decide to either release the photos or take the case to the Supreme Court.

Lawyers for the Justice Department and the Defense Department decided not to appeal the case to the high court, Pentagon spokesman Bryan Whitman told reporters.

"This case had pretty much run its course. The legal options at this point had become pretty limited," Whitman said.

ACLU staff attorney Amrit Singh said on Thursday "these photographs provide visual proof that prisoner abuse by US personnel was not aberrational but widespread, reaching far beyond the walls of Abu Ghraib."

The Iraqi prison became infamous after photographs showing Iraqi detainees being humiliated and abused by their US guards were published in 2004, prompting outrage around the world.

In addition to at least 44 photos cited in the court case, a "substantial number of other images" were also being processed for release, the Justice Department wrote in a court document.

The Pentagon rejected the ACLU's portrayal of the photos as showing widespread abuse of detainees.

"What this demonstrates is that we have always been serious about investigating credible allegations of abuse," Whitman said.

The department's policy had always required "humane treatment" of detainees and those who had violated that policy had been investigated and disciplined, he said.

Rights groups, however, charge senior officials in the Bush administration who were responsible for promoting tactics widely seen as torture have yet to be held accountable.

The Bush administration had refused to release the images to the public, arguing that the disclosure would fuel outrage and violate US obligations toward detainees under the Geneva Conventions.

The decision on the photos comes after the Obama administration released four sensitive memos last week that blew the lid on harsh CIA terror interrogations approved during the Bush era, including the use of insects, simulated drowning and sleep deprivation.

Defense Secretary Robert Gates said on Thursday it was "unrealistic" to try to keep the interrogation memos secret.

In his remarks at a marine base in North Carolina, Gates also made a reference to the lawsuit over the detainee photos, suggesting that hiding information about past abuse would prove difficult.

"There are a number of suits that we're dealing with for detainee photographs and so on," Gates said.

"And so there is a certain inevitability, I believe, that much of this will eventually come out. Much has already come out," Gates said.

Obama has said that CIA officers involved in interrogations should not be prosecuted as they were acting on orders. But he has left the door open to possible prosecution of senior figures in the previous administration.

The president faces demands from rights groups and some fellow Democrats to go after top officials in the Bush administration for their role in the abuse and torture of detainees.

Republican lawmakers fiercely oppose such a move, warning of an imminent "witch hunt" and arguing former officials were trying to keep the country safe from terrorist attack.