A Guantanamo detainee's feet are shackled to the floor as he attends a "Life Skills" class at Guantanamo Bay U.S. Naval Base Thomson Reuters GENEVA (Reuters) - The U.N. torture investigator accused the United States on Wednesday of stalling on his requests to visit its prisons where 80,000 people are in solitary confinement and to interview inmates at Guantanamo on his terms.

Juan Mendez, special rapporteur on torture, has sought for more than two years to enter U.S. prisons, including maximum-security facilities, to check on conditions. U.N. human rights experts have asked to visit Guantanamo since 2004.

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

"Solitary confinement seems to be a permanent feature at both the pre-trial level and post-conviction."

An estimated 80,000 people are in solitary confinement in U.S. prisons, the American Civil Liberties Union (ACLU) says.

"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," Mendez said.

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Solitary confinement tends to be "somewhat moderated", 22 or 23 hours, rather than 24, Mendez said. Inmates are allowed to have reading and writing materials, and television and radio.

"But it's still no meaningful social contact," he said.

U.S. officials have pursued his request, but he said he had not had replies from authorities where he wants to visit state prisons - California, New York, Louisiana and Pennsylvania.

"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."

Activists have called for Washington to open up U.S. prisons to U.N. scrutiny, including Guantanamo in Cuba where they say 122 security detainees remain, including 76 from Yemen.

"It's simply outrageous that it is taking a long time to provide access to U.S. places of detention," Jamil Dakwar, director of the ACLU Human Rights Program, told Reuters.

"Overuse of solitary confinement in the United States is cruel and shameful but hiding or denying it just makes it worse."

Mendez said the terms offered by U.S. authorities to visit Guantanamo were unacceptable, leading him to decline, as did a predecessor in 2004.

"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."

(Reporting by Stephanie Nebehay; editing by Ralph Boulton)