Detainees in US custody at Guantanamo Bay risk developing "irreversible psychiatric symptoms", UN human rights investigators have warned.
UN inspectors say even the exact number of detainees is unknown
They argue conditions at the prison - especially long-term solitary detention - may endanger inmates' mental health.
The six experts noted certain positive moves at Guantanamo in 2004, including some detainees' release, but said these failed to dispel "serious concerns".
The US has said it treats prisoners in accordance with the Geneva Conventions.
The UN inspectors' statement follows widespread allegations of abuse at the US naval base in Guantanamo Bay, Cuba.
It expressed concern the US had done too little to respond to claims of "inhuman and degrading treatment" at the camp, particularly with regard to interrogation methods.
As a result, the investigators said, the US government risked doing more harm than good in the fight against global terror by failing to uphold basic legal rights.
Many of the 540 or so inmates at Guantanamo have been held without charge and access to lawyers since the 2001 invasion of Afghanistan.
They are suspected members of al-Qaeda or the Taleban and the US government accuses them of being "enemy combatants".
The UN inspectors pointed out that many have been held virtually incommunicado for nearly three years, without legal advice or information about the length of their detention.
"The conditions of detention, especially of those in solitary confinement, place the detainees at significant risk of psychiatric deterioration, possibly including the development of irreversible psychiatric symptoms," they said.
The investigators, from Algeria, Canada, New Zealand, Austria, Argentina and Egypt, have twice sought permission to visit Guantanamo to examine the legal aspects of detention.
The US naval base at Guantanamo Bay holds around 550 inmates
The US has not yet agreed but "has indicated an interest in establishing a dialogue with the experts to consider the possibility of a visit," they said.
The legal basis for the prisoners' continued incarceration remains "unclear", the team added, and even the exact number and names of those held are unknown.
Recent moves to accord prisoners more legal rights, following a rebuke by the Supreme Court last year, were "insufficient to dispel the serious concerns" over conditions, the inspectors said.
An array of legal challenges and reviews continues in the US courts, following a US Supreme Court ruling last June that the detainees should have access to the US court system to challenge their imprisonment.
The government claims the Geneva Conventions do not apply to the detainees because they are "enemy combatants", not prisoners of war.
It has promised to investigate fully all allegations of prisoner abuse.