“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 the is issue,” the experts said in a written statement.

The four – Leandro Despouy, rapporteur on the independence of judges and lawyers, Leila Zerrougui, an expert on arbitrary detention, and Paul Hunt and Manfred Nowak, who work for the UN’s health and torture watch dogs – have been assigned to report to the UN Human Rights Commission on respect for international human rights treaties.

They have been trying to gain access to the more than 500 detainees held at Guantanamo Bay since 2002, claiming they have received “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 (US) government documents,” the experts’ statement said.

Revelations in 2004 of abuses at Iraq’s Abu Ghraib prison by US personnel prompted a fresh request to visit.

A spokeswoman for the US mission to the United Nations offices in Geneva told the Associated Press news service that the delayed response is due to a “thorough and independent’ review process.

Back in Washington, the US Defence and State departments have been passing the buck over who has ultimate authority to grant or deny the request.

A Defence Department spokesman reportedly told the CNN news service that the State Department would be responsible for handling the matter.

But that position was contradicted by deputy spokesman for the State Department, Adam Ereli, who said “as far as the specifics of this request go, and as far as arranging the visit, and what’s involved there, and what considerations are in play, and why it has taken a year, I refer you to the Defence Department.”

Meanwhile, the American ABC news service has reported new allegations of abuses at Guantanamo Bay, this time involving breaches of medical confidentiality for interrogation purposes.

According to the ABC, the revelations will be published in the July 7 issue of the New England Journal of Medicine.

Georgetown University law professor M Gregg Bloche, who co-authored the report, claims detainees are not afforded medical confidentiality at Guantanamo Bay.

“In addition, in a systematic fashion, medical information was employed by behavioural science consultants to support the interrogation process,” Mr Bloche said.

“It’s clear that the lack of protection of medical confidentiality violated the Geneva (Convention) rules.”

US Vice President Dick Cheney has refuted claims of mistreatment and resisted growing calls for the military prison to be shutdown.

“They’re living in the tropics. They’re well fed. They’ve got everything they could possibly want,” Mr Cheney said in an interview with CNN.

The Pentagon says it is holding about 520 men in Guantanamo Bay, mostly from Afghanistan, of whom only four have been formally charged.