UN human rights monitors say they will not accept a US offer to visit the Guantanamo Bay prison camp unless they are given free access to the prisoners.
The US says it has been force-feeding detainees on hunger strike
The monitors said they could accept some limitations, but not a ban on private interviews with detainees.
The Pentagon, which received the UN request for a visit more than three years ago, said the invitation showed it had "nothing to hide".
About 500 people are currently being held at Guantanamo.
To date, only the International Committee of the Red Cross has been granted direct access to prisoners at the camp in Cuba.
The three monitors said in a statement that they could not accept the exclusion of private interviews as "this would not only contravene the terms of reference for fact-finding missions... but also undermine the purpose of an objective and fair assessment".
The three also said they were disappointed that the visit would only last one day, and that two other UN human rights investigators had been excluded from the tour.
However, they said they were confident the US government would accept their demand to talk privately with detainees.
Human rights activists have criticised conditions at the camp, where several inmates are on hunger strike.
The UN first asked for permission to visit the camp when it opened in January 2002, months after the US-led invasion of Afghanistan which toppled the Taleban regime.
The UN has accused the US of stalling over its repeated requests to visit the camp to look into allegations of human rights abuses.