The United Nations' top investigator on the use of torture accused Washington of dragging its feet on his requested visits to prisons and refusing to give him access to inmates at Guantanamo.
Juan Mendez said he had been waiting for more than two years for the United States to provide him access to a range of state and federal prisons, where he wants to probe the use of solitary confinement.
Mendez told reporters in Geneva he wanted to visit federal prisons in New York and Colorado and state prisons in New York, California and Louisiana, among others.
He said the US State Department had been working to help him gain access to the state prisons, but after two years of discussions he had yet to receive a positive answer.
"And in one of my last conversations they said that federal prisons were unavailable," he said.
"I fully expect the United States to secure invitations from state prisons for me, but also to be able to visit federal prisons as well," he said.
According to Mendez, "it is not rare" for prisoners in the United States to spend 25 to 30 years in solitary confinement, locked up in a cell with no human contact for between 22 and 23 hours a day.
"It's simply outrageous that it's taking such a long time to provide access to American detention facilities," said Jamil Dakwar, head of human rights at the American Civil Liberties Union (ACLU).
"This begs the question: is the United States hiding something?" he wrote to AFP in an email.
According to the ACLU, more than 80,000 people are held in solitary confinement in the United States on any given day.
Mendez said he was particularly concerned about the use of solitary confinement for underage offenders.
Solitary confinement for children "should never happen, even for a single day," he said, pointing out that the punishment, widely considered cruel even for adults, was "particularly harmful for children because of their state of development and their special needs."
Mendez also harshly criticised Washington for not providing him with "acceptable" access to the US military prison at Guantanamo Bay in Cuba, and to the 122 detainees still being held there.
Washington, he said, had invited him to visit the prison camp in 2012, but under "unacceptable" conditions.
