Investigators from the United Nations have accused the US of stalling over their repeated requests to visit detainees at Guantanamo Bay.
The UN wants to investigate allegations of torture
The UN says it has evidence that torture has taken at the prison amid reports that 520 inmates have had mental breakdowns.
The UN said for over a year there had been no response to its requests to check on the condition of detainees.
The Department of Defense told BBC News the UN request is being considered.
"As for the request to visit with detainees, the ICRC [International Committee of the Red Cross] already performs this important role," a statement said.
Manfred Nowak, the UN's special expert on torture, has been given access to many countries, among them, some with very poor human rights records. He said he had expected more openness from the US
"We are very disappointed that a country that always was very... positive about high human rights standards... and which is also reminding other states that they should actually co-operate fully with the special mechanisms of the UN commission on human rights itself is not living up to these standards," he said.
They have been asking to visit Guantanamo Bay as part of an investigation into allegations of human rights abuse at the US-run prison.
The accusations include "serious allegations of torture, cruel, inhuman and degrading treatment", they said.
"The purpose of the visit would be to examine objectively the allegations first-hand and ascertain whether international human rights standards... are being upheld with respect to those detained persons," the UN statement said.
The body says it will begin an investigation into alleged abuses at Guantanamo Bay with or without US co-operation.
After three and half years in detention, only four inmates held that the base have been charged.
The International Red Cross does not publish the findings from their visits.