(Corrects first paragraph to make clear order relates to 21 photos of 29 originally requested; adds dropped 'U.S.' in court title in paragraph four)

NEW YORK, Sept 22 (Reuters) - A federal appeals court on Monday ordered the release of 21 photographs it said depicted prisoner abuse by U.S. soldiers in Afghanistan and Iraq, which rights groups say prove abuse was widespread.

The pictures, which have never been made public and are part of U.S. Army investigative files, were first ordered released, with redaction, in 2006 by U.S. District Judge Alvin Hellerstein after he reviewed them and ruled they were of critical public interest.

Their release was held up while the U.S. Department of Defense appealed, arguing the release would endanger U.S. soldiers and result in an unwarranted invasion of the privacy of the detainees they depict.

On Monday, the Second U.S. Circuit Court of Appeals turned aside those objections.

The 21 pictures were taken at multiple locations by individuals serving in Iraq and Afghanistan. In the photographs, the detainees "were clothed and generally not forced to pose," the ruling said.

"The photographs depict abusive treatment of detainees by United States soldiers in Iraq and Afghanistan," the ruling said.

The order to release the images is part of a Freedom of Information Act suit filed in 2003 by civil rights groups, including the American Civil Liberties Union, over treatment of detainees in Iraq, Afghanistan and Guantanamo Bay.

The ACLU, who originally requested the release of 29 photos, of which Hellerstein ruled 21 related to detainee abuse, said in a statement that the release of the pictures would help deter future abuse.

"These photographs demonstrate that the abuse of prisoners held in U.S. custody abroad was not aberrational and not confined to Abu Ghraib," ACLU attorney Amrit Singh said in a reference to the Iraqi Abu Ghraib prison that gained notoriety in 2004 when photos emerged of U.S. soldiers abusing Iraqi detainees.

"Their release is critical for bringing an end to the administration's torture policies and for deterring further prisoner abuse," Singh said.

A spokeswoman for the U.S. Attorney's Office in Manhattan had no comment. The government could try to appeal at the U.S. Supreme Court but otherwise the photos could be released by the ACLU within weeks.

In response to the suit, the Pentagon initially offered a list of documents, including a separate group of photographs taken in the Abu Ghraib prison, which included scenes of detainees being physically abused and sexually humiliated, but declined to make them public.

Hellerstein said the release of the images, which were provided by Sgt. Joseph Darby whose photos set off the Iraqi prisoner abuse scandal, would initiate debate on the conduct of American soldiers and about the U.S. Army's command structure.

The Pentagon appealed but backed down after the same photos and others were published on Salon.com.

To date more than 100,000 pages of government documents have been released in response to the lawsuit. (Reporting by Edith Honan; Editing by Christine Kearney and Bill Trott)

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