In an 11-page final report on its review of U.S. adherence to the world's anti-torture treaty, the U.N. Committee Against Torture said detainees should not be returned to any state where they could face a "real risk" of being tortured.
"The state party should cease to detain any person at Guantanamo Bay and close the detention facility," said the panel of 10 independent experts.
The committee said it was concerned that detainees were being held for protracted periods with insufficient legal safeguards and without judicial assessment of the justification for their detention.
The committee was also concerned about allegations that the United States has established secret prisons, where the international Red Cross does not have access to the detainees. The report did not specifically say that such prisons existed, but stated the United States "should ensure that no one is detained in any secret detention facility under its de facto effective control.
Washington should also "investigate and disclose the existence of any such facilities and the authority under which they have been established and the manner in which detainees are treated."
But the U.S. does not have a legal obligation to respond, said CBS News foreign affairs analyst Pamela Falk.
"It is not enforceable, but the conclusion of the report — that the U.S. should close the detention facility — is a reflection of the view of both allies and adversaries that many practices at Guantanamo violate basic human rights," Falk said.
The report also said the United States must "eradicate" all forms of torture committed by military or civilian personnel in Afghanistan, Iraq and other places of detention under its control and investigate allegations thoroughly, prosecuting any staff found guilty.
It pointed in particular to "any interrogation technique — including methods involving sexual humiliation, 'water boarding,' 'short shackling' and using dogs to induce fear."
Water boarding is a controversial technique in which a subject is made to think he is drowning. Short shackling involves shackling a detainee to a hook in the floor to limit movement.
The panel was also concerned that the United States was sending suspects, without judicial review, to states where they may be tortured, a process known as "extraordinary rendition."
The United States made in its first appearance in six years before the U.N. Committee Against Torture earlier this month, addressing a series of issues ranging from Washington's interpretation of the absolute ban on torture to its interrogation methods in prisons such as Abu Ghraib, Iraq, and Guantanamo.
The panel said some techniques "have resulted in the death of some detainees during interrogation" and criticized vague U.S. guidelines that "have led to serious abuse of detainees."
U.S. officials in Geneva declined to comment immediately Friday.
But U.S. State Department legal adviser John B. Bellinger III, who led the U.S. delegation at the U.N. panel hearing earlier this month, told the experts that all American officials are "prohibited from engaging in torture at all times and in all places."
Bellinger said that most of the "regrettable incidents or allegations" of detainee mistreatment had occurred several years ago and that laws, training and monitoring have since been improved.
There have been about 800 investigations into allegations of mistreatment in Afghanistan and Iraq, the U.S. delegation said. The Department of Defense found misconduct and took action against more than 250 service personnel; there have been 103 courts martial and 89 service members were convicted, of whom 19 received sentences of one year or more.
The panel asked the United States to report back within a year with its response to several of its concerns and recommendations.
These include the panel's concerns about secret prisons, extraordinary rendition and the use of interrogation techniques that have resulted in deaths.
"We hope that the United States takes to heart these criticisms and recommendations and begins a significant shift in its policies, including at a minimum immediate access to the prisoners that are in secret detention facilities," Jennifer Daskal of Human Rights Watch told The Associated Press.
But closing the prison would not be simple, Falk said.
"The issue of closing Guantanamo may be as complicated as its creation, because returning prisoners or detainees to a country where they might be tortured is contrary to international law, regardless of where they are being held," Falk said.
The committee "has clearly rejected the United States position that it can shield its civilian CIA operations from the scrutiny of the committee," Daskal added.