The UN special rapporteur on torture, Manfred Nowak, who earlier revealed that he was hoping to be able to visit the US military detention centre at the Guantanamo naval base in Cuba, said secrecy effectively ensured that detainees could disappear.
"It is of concern. When I said that I would like to visit all detention facilities under the US authority where people suspected of terrorism are held I mean not only the few ones that are known," Nowak told journalists.
"I also mean those that are not known, because there is an obligation not to keep secret places of detention, which means making people disappear," he added.
Media reports have indicated there are other isolated secret US detention facilities for terror suspects in other parts of the world.
The Washington Post also reported last month that some prisoners at the notorious Abu Ghraib prison in Iraq were being kept in special cells, out of sight from inspection visits by the International Committee of the Red Cross (ICRC).
Nowak said he had raised the issue with Washington but had not received a response, and insisted that the US government "has a responsibility to provide information on that".
"I made it very clear that if I`m invited, I should have access to all areas within the detention facilities and to speak privately to all detainees is a condition sine qua non, otherwise it doesn`t make sense to go there," Nowak said.
Nowak insisted a day after he met US officials in Geneva about visiting Guantanamo Bay that a mission there with unfettered access to prisoners must go ahead swiftly.
"They understood we can`t postpone this issue anymore. Either we get an invitation for this year or probably not. It is very urgent," the UN expert said.
"We are not interested to wait until all the persons from Guantanamo have been released or sent to their country of origin," he added.
Only the ICRC has been allowed to make confidential visits in Guantanamo to check on conditions there while other agencies have been rebuffed.
In a joint declaration published in early February, six UN human rights experts including Nowak expressed their concern about the well-being of the prisoners, saying that conditions at the detention facility put detainees at risk of "irreversible" psychiatric deterioration.
US military command began an investigation in January after the publication of e-mails from FBI agents to their superiors recalling incidents of physical abuse at Guantanamo.
The UN expert also reiterated criticism of US Attorney-General Alberto Gonzales`s advice to the administration -- made before he was appointed -- about the applicability of rules against mistreatment to foreign citizens outside US territory.
"As soon as you make this distinction, you`re opening Pandora`s box. You`re not allowed to torture a person, even if you would save hundreds of lives," Nowak emphasised.