A United Nations diplomat charged with investigating claims of torture said Monday that he is “deeply disappointed and frustrated” that U.S. defense officials have refused his request for an unmonitored visit with Pfc. Bradley Manning, the Army intelligence analyst accused of passing classified material to WikiLeaks.

Juan E. Mendez, the U.N. special rapporteur on torture, said his request for a private interview with Manning was denied by the Defense Department on Friday. Instead, he has been told that any visit must be supervised.

Mendez has been seeking to determine whether Manning’s confinement at a military brig at Quantico amounts to torture, following complaints about his treatment and an incident in which the private was forced to strip in his cell at night and sleep without clothing.

“My request . . . is not onerous: for my part, a monitored conversation would not comply with the practices that my mandate applies in every country and detention center visited,” Mendez said in a statement Monday, noting that at least 18 countries have allowed unmonitored interviews.

Manning, 23, has been held at Quantico since July 29 and is awaiting a possible court-martial on charges that he endangered national security by allegedly leaking classified military and diplomatic information.

For most of this time, military officials have kept Manning under “prevention of injury” watch, asserting that he poses a risk to himself. That means he spends 23 hours a day alone in his cell, with one hour allowed for exercise, and has no contact with other prisoners. He is allowed visitors for a few hours on the weekends. He must give up his prison uniform at night, though jail officials have now issued him a smock to wear.

U.S. officials have denied that Manning is being mistreated and have said that the circumstances of his confinement comply with U.S. law and Defense Department regulations.

Last month, however, P.J. Crowley, then the spokesman for the State Department, said the conditions of Manning’s confinement were “counterproductive and stupid” — a comment that angered the White House and prompted Crowley’s resignation.

On Sunday, the New York Review of Books published a letter signed by more than 250 lawyers, professors and authors, including Harvard University constitutional law professor Laurence H. Tribe, that called the conditions of Manning’s confinement “illegal and immoral.” The British government has also raised concerns about the issue.

In an interview, Mendez said that “at first glance,” Manning’s case seems to be “of interest to my mandate,” which is to investigate cases of cruel, inhumane or degrading treatment and report them to the U.N. Human Rights Council.

To do his job, he said, he needs to be able to speak to Manning without witnesses, including guards patrolling nearby. Otherwise, he said, “I cannot be sure Manning is being absolutely candid and honest with me if he knows that he’s being monitored.”

He said he is willing to see Manning nonetheless, if Manning wishes to see him.

The Defense Department has also denied requests for unmonitored visits with Manning by a representative of Amnesty International and by Rep. Dennis J. Kucinich (D-Ohio), according to the soldier’s attorney.