BERLIN (Reuters) - Over two-thirds of the detainees in the Guantanamo Bay prison are suffering from or at risk of mental problems because they are kept isolated in small cells with little light or fresh air, according to Human Rights Watch.

A detainee is escorted by U.S. Navy guards in Camp Four, the facility containing the "most compliant" detainees, at the Guantanamo Bay Naval Station in Guantanamo Bay, September 4, 2007. REUTERS/Joe Skipper

In a report entitled “Locked Up Alone: Detention Conditions and Mental Health at Guantanamo”, the group says 185 of the 270 detainees at the U.S. military prison for terrorism suspects are housed in facilities similar to “supermax” prisons.

They spend 22 hours alone in cramped cells, have very limited contact with other human beings and are given little more than the Koran to occupy themselves, said the report, which is based interviews with government officials and attorneys.

Detainees held in this manner include many that have not been charged with crimes and have already been cleared for release or transfer, according to the report.

“Guantanamo detainees who have not even been charged with a crime are being warehoused in conditions that are in many ways harsher than those reserved for the most dangerous, convicted criminals in the United States,” said Jennifer Daskal, senior counterterrorism counsel at Human Rights Watch.

More than six years after the United States began sending terrorism suspects to the naval base in Cuba, not a single case has gone to trial.

Nineteen cases are now pending, including some that have been delayed repeatedly amid challenges to the legality of Guantanamo war crimes court set up by the administration of President George W. Bush.

Both U.S. presidential candidates, Democrat Barack Obama and Republican John McCain, have pledged to close the prison, which has been denounced by human rights groups and foreign governments for years.

The Bush administration denies that Guantanamo prisoners are treated inhumanely and the president has said he would like to close the detention centre.

The Human Rights Watch report says that even the two hours of recreation time afforded the prisoners in Guantanamo generally takes place in single-cell cages so that detainees cannot physically interact with one another.

Unlike prisoners in most high security prisons in the United States, none of the Guantanamo detainees have been allowed visits by family members and very few have been able to make phone calls home, the report says.

Several are reportedly suffering depression and anxiety disorder, and some have reported having visions and hearing voices, Human Rights Watch said.