The four independent experts said the United States had not responded to their numerous requests to check on the conditions of terror suspects at the U.S. Naval base in Cuba, as well as U.S. facilities in Iraq, Afghanistan and other places of detention.
They said their request for a visit was "based on information, from reliable sources, of serious allegations of torture, cruel, inhuman and degrading treatment of detainees, arbitrary detention, violations of their right to health and their due process rights."
"Many of these allegations have come to light through declassified (U.S.) government documents," they said.
A U.S. spokeswoman said the request was still being reviewed in Washington.
"Credible concerns about treatment of detainees at Guantanamo have been voiced by the International Committee of the Red Cross and other human rights organizations," said CBS News Foreign Affairs Analyst Pamela Falk from the United Nations. "But the current request for on-site visits comes out of a U.N. Commission on Human Rights committee of four experts and has grave political overtones since it was presented by Cuba in Geneva last month and because of the credibility problems with the Commission itself."
"Most of the proposals for reform of the United Nations seek to overhaul or scrap the Commission on Human Rights because its membership has consistently included some of the world's most persistent human rights violators," added Falk. "The new High Commissioner is in office just one week because of the resignation of the previous one after sexual harassment allegations. The Commission simply does not bring added value to the search for solutions in Guantanamo."
"We are all worried about this situation," Leandro Despouy of Argentina, specialist on the independence of judges and lawyers, told reporters.
The expert on torture, Manfred Nowak of Austria, said, "We are not making a judgment if torture or treatment under degrading conditions has taken place."
However, he said, the failure of the United States to respond is leading the experts to conclude that Washington has something to hide.
"At a certain point you have to take well-founded allegations as proven in the absence of a clear explanation by the government."
Nowak said the experts have received a number of allegations about abuse of U.S. detainees.
The experts, who report to U.N. bodies on different human rights issues, said they had yet to hear back from Washington on their latest request — made a year ago and renewed in mid-April — to visit the detention facility.
The U.N. human rights investigators have been trying to visit Guantanamo since early 2002.
Nowak said that in a meeting in April U.S. officials refused to guarantee him the right to speak to detainees in private — an "absolute precondition" for such a visit.
Nowak said his team would need full access to the facilities and the prison population.
"We deeply regret that the government of the United States has still not invited us to visit those persons arrested, detained or tried on grounds of alleged terrorism or other violations," said the statement from the experts.
"The time is up. We have to act now," said Leila Zerrougui of Algeria, specialist on arbitrary detention. "If not, we won't have any credibility left."
Paul Hunt of New Zealand, who monitors physical and mental health, said he wanted to investigate in person "persistent and credible reports" of alleged violations.
"Reportedly medical staff have assisted in the design of interrogation strategies, including sleep deprivation and other coercive interrogation methods," said Hunt.
The experts said they decided to express their misgivings because "the lack of a definitive answer despite repeated requests suggests that the United States is not willing to cooperate with the United Nations human rights machinery on this issue."
Brooks Robinson, spokeswoman for the U.S. mission to U.N. offices in Geneva, said, "U.S. policy prohibits and condemns torture."
"American personnel are required to follow this policy and applicable law," she told The Associated Press. "Credible allegations of illegal conduct by U.S. personnel are taken seriously and investigated."
Robinson said the U.S. response to the experts had been delayed because of the United States' review process which is "thorough and independent," and involves the Bush administration, Congress and the U.S. judicial system.
"It is true there is no answer yet to their request, but the main point is that their request is being addressed and discussed and reviewed in the United States," Robinson said.
"That process is underway in response to this request," she added, noting that U.S. officials have consistently denied violating the principle of humane treatment of detainees in the war on terror.
The experts said no country is above international human rights law.
"The writ of international human rights does not stop at the gates of Guantanamo Bay," said Hunt.
U.S. officials so far have allowed only the International Committee of the Red Cross to visit detainees at Guantanamo, which began being used as a detention center for terror suspects with the arrival of the first alleged Taliban and al Qaeda members brought from Afghanistan in early 2002.
The ICRC keeps its findings confidential, reporting them solely to the detaining power, although some of the reports have been leaked by what the ICRC says were third parties. The U.N. experts would be expected to make a public report.
The four experts have all been appointed to their three-year terms by the 53-nation U.N. Human Rights Commission, the global body's top rights watchdog. They act independently and are unpaid by the United Nations for their work although their expenses are paid.