The photographs were some of thousands turned over by Specialist Joseph M. Darby, the whistle-blower who exposed the abuse at Abu Ghraib by giving investigators computer disks containing photographs and videos of prisoners being abused, sexually humiliated and threatened with growling dogs.

The small number of the photographs released in spring 2004 provoked international outrage at the American military.

In early June, Judge Alvin K. Hellerstein of Federal District Court in Manhattan ordered the release of the additional photographs, part of a Freedom of Information Act lawsuit filed by the American Civil Liberties Union to determine the extent of abuse at American military prisons in Iraq and Afghanistan and at Guantánamo Bay, Cuba.

The government has turned over more than 60,000 pages of documents on the treatment of detainees, some containing graphic descriptions of mistreatment. But the material that the judge ordered released - the A.C.L.U. says there are 87 photographs and 4 videos - would be the first images released in the suit. The judge said they would be the "best evidence" in the debate about the treatment of Abu Ghraib prisoners.

"There is another dimension to a picture that is of much greater moment and immediacy" than a document, Judge Hellerstein said in court.

He rejected arguments from the government that releasing the photographs would violate the Geneva Conventions because prisoners might be identified and "further humiliated," but he ordered any identifying features to be removed from the images.

In the letter sent Thursday, Sean Lane, an assistant United States attorney, said that the government was withholding the photographs because they "could result in harm to individuals," and that it would outline the reasons in a sealed brief to the court.

The A.C.L.U. accused the government of continuing to stonewall requests for information "of critical public interest."

"The government chose the last possible moment to raise this argument," said Amrit Singh, a staff lawyer with the A.C.L.U.

"Because it is under seal, we don't know whether their reasons are adequate," Ms. Singh said.