A UN investigator on torture says US officials have blocked his requests for unmonitored visits to alleged Wikileaks suspect Bradley Manning.
Juan Mendez said he was "disappointed and frustrated by the prevarication" he met from US officials.
Pte Manning, accused of leaking secret military and diplomatic cables, has been detained at a US marine base since May 2010.
The 23-year-old's lawyers claim he has faced mistreatment at the base.
The US government "has not been receptive to a confidential meeting with Mr Manning", said Mr Mendez in a statement.
He suggests the authorities will only allow him to visit Pte Manning at the Quantico Marine base in Virginia if officials are present.
Mr Mendez, the UN special rapporteur on torture, said a monitored conversation would be counter to his UN mandate.
It would not allow him to verify whether the detainee had been ill-treated, he added.
Supporters of Pte Manning say he has been held under harsh conditions, confined to a spartan cell for for 23 hours a day without personal possessions, and forced regularly to undress.
Pentagon spokesman Colonel Dave Lapan said that only lawyers were allowed confidential meetings with anyone confined at Quantico, reports news agency Reuters.
Col Lapan also said there had been "considerable misinformation" about Pte Manning's confinement.
Apart from a brief period about a month ago, the accused had not been forced to sleep naked or awakened repeatedly, Col Lapan added.
Pte Manning is awaiting trial on charges that while serving as a US army intelligence analyst in Iraq he leaked 720,000 secret US documents.
Last month, state department spokesman PJ Crowley resigned after he said US treatment of Pte Manning was "stupid".