The Pentagon has invited UN officials to visit the Guantanamo Bay prison camp, more than three years after first receiving the request.
The US says it has nothing to hide
Three human right monitors will be allowed to observe the facilities and question military officials but will not have access to detainees.
The Pentagon said the invitation showed it had "nothing to hide".
Human rights activists have criticised conditions at the camp in Cuba where several inmates are on hunger strike.
In a statement, the Pentagon said it strived to be as open as possible while taking into consideration "security and operational requirements and the need to ensure the safety of our forces".
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.
About 505 prisoners remain at Guantanamo, many of them captured in Afghanistan.
A Pentagon spokesman, Lt Col Mark Ballesteros, said the UN officials would not be allowed to speak to detainees because that was the role of the International Committee of the Red Cross.
The UN has accused the US of stalling over its repeated requests to visit the camp to look into allegations of human rights abuses.
Human rights campaigners have expressed growing concern about the treatment of the inmates, some of whom are on hunger strike in protest about conditions.
The Pentagon says 26 detainees have been on a hunger strike that began in August but UK human rights charity Reprieve said 200 have taken part in the protest in cycles.
The US has provoked controversy with its policy to force-feed the hunger strikers. Campaigners said the policy was unethical and painful but the Pentagon said it was saving lives.