In a letter to the Senate Armed Services Committee, the Pentagon confirmed on Wednesday that a new disc contained 11 pictures that had not yet made it to the international press.
But the assistant defence secretary for legislative affairs, Powell Moore, said the images "may not be original or true photographs".
He said they were given to the Criminal Investigation Command in Baghdad "under circumstances that warranted investigation, including forensic computer evaluation."
In separate photos shown on ABC News, privates Sabrina Harmon and Charles Graner are seen smiling and giving a "thumbs-up" sign over the body of a man identified as Manadil al-Jamadi, an Iraqi who died in US custody at the prison.
ABC reported on its Web site that the photos were taken by Sergeant Charles Frederick, who in e-mails to his family questioned why those responsible for the prisoner's death were not being prosecuted in the same manner that he is.
US congressmen have seen
more than 1600 abuse images
Frederick, Harmon and Graner are among six US military police facing charges over the abuse of detainees.
A seventh soldier, Jeremy Sivits, was sent to jail and dismissed from the Army on Wednesday after pleading guilty in the first abuse trial.
Members of the US Congress saw more than 1600 images from the investigation last week, which they said showed shocking examples of abuse and sexual humiliation of prisoners.