U.N. human rights investigators, citing "persistent and credible" reports of torture at the U.S. base in Guantanamo Bay, urged the United States on Thursday to allow them to check conditions there.

The failure of the United States to respond to requests since early 2002 is leading the experts to conclude Washington has something to hide at the Cuban base, said Manfred Nowak, a specialist on torture and a professor of human rights law in Vienna, Austria.

"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.

However, he added: "We are not making a judgment if torture or treatment under degrading conditions has taken place."

Washington's response is delayed because the U.S. review process is "thorough and independent" and involves the Bush administration, Congress and the judicial system, said Brooks Robinson, spokeswoman for the U.S. mission to U.N. offices in Geneva.

"The main point is that their request is being addressed and discussed and reviewed in the United States," Robinson said. "That process is underway."

But one investigator, Algerian magistrate Leila Zerrougui, said: "The time is up. We have to act now. If not, we won't have any credibility left."

For more than three years, U.N. investigators have made numerous requests to visit foreign terror suspects at the U.S. Naval base in Cuba and at U.S. facilities in Iraq, Afghanistan and elsewhere, the four independent specialists told reporters.

"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," the experts said.

The four, who report to U.N. bodies on different human rights issues, are appointed to their three-year terms by the 53-nation U.N. Human Rights Commission, the global body's top rights watchdog. They are unpaid for their work, although their expenses are paid.

The United States has criticized the commission because its members include countries with tyrannical governments and poor human rights records, but the experts operate autonomously, often reproaching their own countries and others in the commission.

The four cited "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," the statement said.

U.S. officials have consistently denied violating the principle of humane treatment of detainees in the war on terror, and Robinson noted that American policy "prohibits and condemns torture."

"American personnel are required to follow this policy and applicable law," she said. "Credible allegations of illegal conduct by U.S. personnel are taken seriously and investigated."

The specialists 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.

Source: CD/Agencies