A statement by UN investigators highlighted the dangerous side effects of solitary confinement, among other detention techniques.
"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," the joint statement by investigators into the detention centre said on Friday.
The United States continues to hold about 550 suspects from more than 20 countries at the centre.
Noting that there is no longer any international conflict in Afghanistan or Iraq, the statement quoted the Third Geneva Convention, which states that prisoners of war must be released without delay when hostilities end.
The statement also noted that the exact number and names of people held at Guantanamo remained unknown to the outside world.
"Many of the inmates are completing their third year of virtually incommunicado detention, without legal assistance or
information as to the expected duration of their detention, and in conditions of detention that, according to numerous observers, amount to inhuman and degrading treatment," the statement said.
"This situation is extremely disconcerting and is conducive to the unacknowledged transfer of inmates to other, often secret, detention facilities, whether run by the United States or by other countries," it said.
"The conditions of detention, especially of those in solitary confinement, place the detainees at significant risk of psychiatric deterioration"
Several experts had approached the US, Afghan and Iraqi authorities to try to visit the facility and interview prisoners. While the United States government had not yet agreed to the request, it had indicated an interest in establishing a dialogue with the experts to consider the possibility of a visit.
The UN investigators criticised the situation of uncertainty regarding any criminal charges that might be pending.
Legal access denied
Most prisoners had no access to lawyers in proceedings in the
United States. The UN team also expressed doubts about the
independence of the special US military tribunals set up last year by the Pentagon.
They said they were concerned about allegations of torture
Bush's administration calls the
detainees enemy combatants
during interrogations. The US Justice Department on Thursday appealed against a federal court ban on military tribunals for international terror suspects held at the base.
Judge Joyce Hens Green ruled on Monday that the prisoners have rights under the US constitution.
The administration of President George Bush said it would appeal, stating that the prisoners in Guantanamo, are enemy combatants and, therefore, do not have such rights.
On Thursday, the US navy ruled that three more Guantanamo prisoners had been wrongly classified as enemy combatants, using a tribunal process that was found constitutionally flawed.
That brings to six the number found not to be enemy combatants since the United States began sending terrorism
suspects to the Guantanamo Bay naval station in southeastern Cuba three years ago.
A Pentagon spokesman, Lieutenant Commander Daryl Borgquist, would not identify the three or their nationalities but said the US Department of State would arrange for them to go home.
So far, the tribunals have ruled that 387 other Guantanamo prisoners were correctly classified as enemy combatants. Final decisions in 165 cases are still pending before Rear Admiral James McGarrah in Washington, who is reviewing records from administrative hearings that concluded at Guantanamo Bay last month.
The Pentagon created the Combatant Status Review Tribunals after the Supreme Court ruled in July that the Guantanamo prisoners had the right to challenge their indefinite detention.