The US is violating UN rules by refusing unmonitored access to the Army private who is accused of passing secret documents to WikiLeaks, the UN's chief torture investigator has said.
UN special rapporteur on torture Juan Mendez said the US had has broken rules by insisting on monitoring conversations with Pte Bradley Manning.
Mr Mendez says he needs unrestricted access to Pte Manning to do his job.
Pte Manning, 23, is being held in a military prison awaiting trial.
The intelligence analyst, who joined the US Army in 2007, is accused of leaking 720,000 secret military and diplomatic US government documents.
They were later published by the whistle-blowing website Wikileaks.
Advocates for government transparency have called for the released of Pte Manning, placing pressure on the US government.
After being confined alone in a cell for 23 hours per day in a detention facility in Quantico in the state of Virginia, Pte Manning was transferred to Fort Leavenworth military prison in Kansas in April.
Mr Mendez said the US had told him Mr Manning was being treated better now than when he was in Quantico.
But the UN investigator said the US must allow him to determine whether the conditions at Quantico that Pte Manning experienced amounted to "torture or cruel, inhuman or degrading treatment or punishment".
"For that, it is imperative that I talk to Mr Manning under conditions where I can be assured that he is being absolutely candid," Mr Mendez said in a statement.
Mr Mendez said that because the US is a "strong supporter of the international human rights system", the country's actions "must seek to set the pace in good practices that enhance the role of human rights mechanisms, ensuring and maintaining unfettered access to detainees during enquiries".
Pentagon officials have previously said Pte Manning is being held in appropriate conditions considering the seriousness of the charges against him.
He has been charged with using unauthorised software on government computers to download classified information and to make intelligence available to "the enemy", as well as other counts related to leaking intelligence and theft of public records.