"I think sooner or later there will be a need to close Guantanamo," said Secretary-General, Kofi Annan, at the UN headquarters.
"It will be up to the [US] government to decide hopefully to do it as soon as possible," he said.
His comments came after the report, leaked earlier in the week, was released, saying prisoners at the facility have been abused.
Mr Annan said while he does not necessarily agree with everything in the report, "the basic point that one cannot detain individuals in perpetuity and that charges have to be brought against them and they must be given a chance to explain themselves and be prosecuted, charged, or released.”
The White House has blasted the report and called it "a discredit to the UN".
Spokesman Scott McClellan said the world body has not looked into all the facts, only the allegations.
"The United Nations should be making serious investigations across the world, and there are many instances in which they do when it comes to human rights. This was not one of them," he said.
In their report, five independent experts who act as monitors for the UN Human Rights Commission said aspects of the treatment of detainees violate their rights to physical and mental health, and in some cases amount to torture.
The US administration currently "operates as judge, prosecutor and defence counsel of the Guantanamo Bay detainees," it said.
Many of the 500-odd inmates at the US naval base in Cuba have been held for four years without trial.
The report also said the US' justification for holding the inmates is a distortion of international human rights treaties.
The draft version of the report was leaked earlier this week and was rejected by the US as making a "baseless assertion", saying its authors had never visited the prison.
UN human rights experts began talks with the US inn 2002 on a possible visit.
The experts in the report said they cancelled a planned visit in December after failing to get approval to speak freely with prisoners, and in the report demanded full and unrestricted access to Guantanamo.
The findings were based on the US government's answers to a questionnaire as well as interviews with former inmates in Europe and lawyers for current detainees.
EU adds its voice
Shortly after the UN report was made public, European Union legislators also urged the US to close the facility and give a fair trial to all prisoners.
In a resolution passed overwhelmingly at the European Parliament in Strasbourg, France, the European Parliament said: "every prisoner should be treated in accordance with international humanitarian law and tried without delay in a fair and public hearing by a competent, independent, impartial tribunal."
Amnesty International has also backed the call, saying Guantanamo represents "just the tip of the iceberg" of US-run detention facilities worldwide.
The report said detainees have suffered harsh treatment such as solitary confinement, being stripped naked, subjected to severe temperatures and being threatened with dogs, which is banned in all circumstances.
"The excessive violence used in many cases during transportation ... and forced-feeding of detainees on hunger strike must be assessed as amounting to torture," it added.
US response
The US has unequivocally rejected the UN demands.
The White House insisted that the detainees are treated humanely, with new challenges to be made to the US Supreme Court this week and new war crimes trials about to get underway at the camp this month.
"These are dangerous terrorists that we're talking about that are there," he said, adding that "nothing's changed" in the US opinion of whether the camp should close.
He suggested that allegations of abuse amounting to torture at the camp are propaganda by militants trained to make such charges.
"We know al-Qaeda detainees are trained in trying to disseminate false allegations," he said.
Meanwhile, a judge in Britain has granted three British Guantanamo detainees to seek a court order asking the British government to petition for their release.
Judge Andrew Collins at London's High Court said the three have a case and the British government is obliged to act of their behalf, adding that the US idea of what constitutes torture "is not the same as ours ands doesn't appear to coincide with that of most civilised countries".