A senior United Nations representative on torture, Juan Mendez, issued a rare reprimand to the US government on Monday for failing to allow him to meet in private Bradley Manning, the American soldier accused of being the WikiLeaks source and held in a military prison. It is the kind of censure the UN normally reserves for authoritarian regimes around the world.

Mendez, the UN special rapporteur on torture, said: "I am deeply disappointed and frustrated by the prevarication of the US government with regard to my attempts to visit Mr Manning."

Manning's supporters claim that the US is being vindictive in its treatment of Manning, who is held at the marine base at Quantico, Virginia, in conditions they describe as inhumane.

Mendez told the Guardian: "I am acting on a complaint that the regimen of this detainee amounts to cruel, inhumane or degrading treatment or torture … until I have all the evidence in front of me, I cannot say whether he has been treated inhumanely."

Mendez said the vast majority of states allowed for visits to detainees without conditions. But the US department of defence would not allow him to make an "official" visit, only a "private" one. An official visit would mean he meets Manning without a guard. A private visit means with a guard. Also, anything the prisoner says could be used in a court-martial.

Mendez said his mandate was to conduct unmonitored visits. He had met representatives from the state department and the Pentagon on Friday and learned their decision over the weekend.

Although he was prepared to meet Manning with a guard present, he would continue to press for an unmonitored visit.

"I am insisting the US government lets me see him without witnesses. I am asking [the US government] to reconsider," Mendez said.

Colonel Dave Lapan, a Pentagon spokesman, said: "We cannot, under Quantico brig practice, guarantee the UN special rapporteur an unmonitored visit. At Quantico, such a guarantee is only reserved for attorney-client communications. As in the federal prison system, and for security reasons, the department of defence does not guarantee unmonitored communications with confinees except for privileged communications or in other special circumstances not present here."

He added that there was a lot of misinformation about Manning and insisted he was not in solitary confinement.

"There is no such thing at Quantico. PFC Manning is in maximum security, which does not affect the type of cell he is in. He occupies the same type of single-occupancy cell that a medium security confinee at Quantico would occupy, in the same general area of the brig that a medium security confinee may occupy. Except for a brief period about a month ago, and for reasons of Manning's own physical safety, Manning does not sleep naked. Nor is Manning awakened every five minutes by brig personnel. These facts are simply not true.

"Manning is allowed to receive visitors, receive and send mail, watch TV, exercise outside his cell, and visit with doctors and mental health providers."