The United Nations torture investigator said the United States has for two years blocked his attempts to visit its prisons, where 80,000 people are held in solitary confinement, and to speak freely with Guantanamo Bay inmates.

Juan Méndez said Wednesday that he has tried for more than two years to visit the U.S. and check on the conditions of American prisons, including maximum-security prisons, Reuters reported. Mendez added that U.N. human rights officials have asked to visit Guantanamo Bay prisoners since 2004.

"On the federal level, I want to go to ADX in Florence, Colorado and to the Manhattan Correctional Center," Méndez said during a news briefing. "Those are where people accused of terrorism are taken or where they serve their term."

Of particular interest to Mendez are the 80,000 people being held in solitary confinement in U.S. prisons. In 2011, he called for a ban on the practice of solitary confinement, as he said it is used to break down a prisoner's psychological will.

"Solitary confinement seems to be a permanent feature at both the pre-trial level and post-conviction," Méndez said. "The numbers are staggering but even worse is the length of terms...It is not uncommon for people to spend 25, 30 years and even more in solitary confinement."

Méndez said even though solitary is "somewhat moderated" to 22 or 23 hours per day, rather than 24, and inmates are allowed to have reading and writing materials and T.V. and radio, "It's still no meaningful social contact."

U.S. officials have reviewed his request, he said, but state prison officials in California, New York, Louisiana and Pennsylvania have ignored him.

"In one of my last conversations, they said that federal prisons were unavailable. I cannot accept an invitation to say go to a California prison if the federal prisons are off-limits to me," Méndez said.

As for the controversial Guantanamo Bay, Cuba prison, Méndez said he has only been given permission to visit with certain restrictions attached that would limit his inquiry, which he, like his predecessor, declined due to those restrictions being too prohibitive.

"The invitation is to get a briefing from the authorities and to visit some parts of the prison, but not all, and specifically I am not allowed to have unmonitored or even monitored conversations with any inmate in Guantanamo Bay," Mendez said, according to Reuters.

Around 122 detainees are still held at the Guantanamo prison out of the public spotlight, and the majority have never received a trial or even been charged with a crime, according to Human Rights Watch.