UN human rights investigators rejected an invitation to visit the Guantanamo Bay prison camp, saying on Friday Washington would not let them interview the more than 500 people held there.
The envoys, who report on torture, arbitrary detention and other abuses for the United Nations, had warned that they would not go unless Washington let them speak to detained terrorism suspects as well as US prison staff and officials.
This was normal practice on all such visits, they said. But Washington replied that only the International Committee of the Red Cross (ICRC) was permitted to speak directly to detainees.
"We deeply regret that the United States government did not accept the standard terms of reference for a credible, objective and fair assessment of the situation of the detainees," the five UN investigators said in a statement.
"Under the circumstances, we will not be travelling to Guantanamo Bay naval station," they added.
Washington defends its treatment of prisoners and denies that torture has occurred at the Guantanamo facility, which opened in January 2002, just months after the September 11, 2001, attacks on the United States. Most of the detainees were seized in Afghanistan.
Human rights activists have criticised jail conditions and the indefinite detention without trial of suspects at the naval base in Cuba. Only nine have been charged so far with any crime.
Of the five envoys, Washington invited only three - Austria's Manfred Nowak, special investigator on torture; Pakistan's Asma Jahangir, who focuses on religious freedom; and Algeria's Leila Zerrougui, who looks into arbitrary detention.
It did not accept Argentina's Leandro Despouy, special investigator on the independence of judges and lawyers, and New Zealand's Paul Hunt, special rapporteur on mental and physical health, who were included in the envoys' request.
The others said they would still accept the invitation, extended in October nearly four years after first attempts to visit were made, providing Washington agreed to the interviews.
But Washington stood by its refusal.
"It was something that was looked at quite closely by the US government, and the United States government made a decision in terms of what type of access is appropriate for these rapporteurs," said Pentagon spokesman Bryan Whitman.
It was enough that the United States allowed the ICRC access. The ICRC kept its findings confidential, while the UN envoys would make theirs public, he added.
Speaking in London on Friday, Novak said the envoys had not given up all hope of going to Guantanamo at some stage.
"We continue to hope that the authorities will grant us access ... on reasonable terms," he told journalists while attending an Amnesty International conference on Guantanamo and other camps for US-held prisoners.
But having access to detainees was "the minimum standard of objective fact-finding."
The Geneva director of the US-based group Human Rights Watch said the envoys had no alternative but to turn down the US offer.
"It was impossible. They (Washington) were saying that you can go but you cannot do your job," Loubna Freih told Reuters.