Pfc. Bradley Manning has been in U.S. custody since May 2010 and isolated to a cell at a Marine brig for 23 out of the 24 hours in a day. Manning is accused of handing over sensitive cables to Web site Wikileaks.
Juan Mendez, an independent special envoy to the United Nations on torture, said in a statement that Washington won't let him visit Manning in private despite making repeated requests to the U.S. government since December.
Mendez said he was "deeply disappointed and frustrated" with Washington's decision.
"My request for a private, confidential and unsupervised interview with Manning is not onerous," he said in his statement. "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 it was normal for someone in his position to have private conversations with detainees suspected of being subjected to ill-treatment. Critics say Manning is allegedly subjected to cruel treatment while in detention in Virginia.
The special envoy said Pentagon officials agreed to supervised visits with Manning, though Mendez argues those in his position have carried out private visits with detainees in more than a dozen countries during the past six years.
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