A former US official has reportedly seen several nude photos, including some he described as "very gruesome," of terror suspects snapped by the CIA before the captives were handed off to foreign partners for torture. The photos indicate that sexual humiliation was part of the CIA's strategy to subjugate terror suspects.

In some of the photos, which remain classified, the captives — almost all of them men — are blindfolded, bound and show visible bruises. Some photographs also show people believed to be CIA officials or contractors posing alongside the naked detainees, the American source who has examined the images told the Guardian.

It's unknown how many suspects the CIA held in undisclosed "black sites" overseas in what the agency has called "extraordinary renditions" — or how many detainees were photographed in this way. Some agents have said that stripping detainees was a way to document their condition when they left CIA custody, the Guardian reports.

Clearly, however, that was not the case in several instances. In photos from the infamous Abu Ghraib prison site in Iraq, for example, nakedness was part of the humiliation captives were subjected to as part of their treatment, which also included sex abuse. Naked detainees in the photos were menaced by dogs, made to wear dog collars, piled on top of one another, and paraded naked past US military servicewomen.

"Photographing or videotaping detainees in US custody unrelated to the processing of prisoners or the management of detention facilities can constitute a violation of the laws of war, including the Geneva conventions, in some cases," Nathaniel Raymond, a researcher at the Harvard Humanitarian Initiative, told the Guardian.

The Senate's investigation into CIA torture that was released in 2014 kept the rendition programme secret, but did reveal that the agency photographed detainees before they were handed off to foreign partners. The report, however, did not describe the nature of the photos.

But the probe did reveal that the CIA "routinely" stripped its own detainees nude, although Justice Department officials did not formally approve the practice until 2005. Often the nudity occurred along with other torture techniques, such as shackling and frigid conditions.

Earlier in 2016 the Pentagon released nearly 200 photos showing abuse of prisoners in Iraq and Afghanistan following a Freedom of Information Act lawsuit by the American Civil Liberties Union. It refused, however, to release hundreds of additional photos, arguing that they were so troubling that their exposure could incite an attack on America. The CIA refused to comment on the Guardian's story, the newspaper reported.