Authors of a report commissioned by the U.N. claiming that detainees in Guantanamo Bay, Cuba, are being tortured by American military personnel failed to visit the prison, despite an invitation from the American authorities.

“Any report that they may be writing would certainly suffer from the opportunity that was offered to them to go down there and witness firsthand the operations at Guantanamo,” a Department of Defense spokesman, Bryan Whitman, told reporters at the Pentagon yesterday.

The U.N. representatives declined the invitation to visit Guantanamo because they were told that they would not be able to interview detainees during the visit, Mr. Whitman said. The 500 suspected terrorists, who are being kept without trial after being captured in Afghanistan and elsewhere, are regularly interviewed by representatives of the International Committee of the Red Cross, which does not report on the treatment of prisoners.

Members of investigation, who will deliver their report later this year to the U.N. Commission on Human Rights, have spent 18 months investigating conditions at Guantanamo and have relied for information about the condition of prisoners upon anecdotes provided by released detainees and family members and lawyers of those who are or have been detained from France, Spain, and Britain. Information has also been gleaned from the Department of State.

The report is currently in draft form but has been widely leaked, suggesting the final version will accuse the American military of maltreatment of prisoners amounting to torture and will demand closure of the prison.

The American government considers the failure of U.N. representatives to see for themselves conditions at the prison camp a damning omission from a body that has already been discredited. When U.N. secretary general Kofi Annan, who said he has not read the draft report, raised Guantanamo with President Bush in their meeting on Monday, the president dismissed the topic.

“When people hear these press reports about these outcomes and when they actually view the final report, I would urge them to look at it in the context of the fact that nobody who wrote this report actually went to Guantanamo,” a State Department spokesman, Sean McCormack, who denied accusations of torture, said.

“One of the assertions about torture allegedly being committed down in Guantanamo Bay centered around the use of feeding those prisoners who had gone on hunger strikes,” Mr. McCormack said. “I have to tell you that the doctors down there comply with accepted international practice when it comes to these questions.”

He insisted the detainees were not innocent victims but perpetrators of acts of terror. “As far as we are concerned, they were picked up for a good reason,” he said. The prison at Guantanamo “is protecting the American people, as well as others, from dangerous individuals,” he said.