The Swiss-run International Committee of the Red Cross (ICRC) says it believes the United States is secretly holding terror suspects in locations around the world.
The Geneva-based organisation claims suspects reported by the US authorities as captured have never turned up in detention centres.
The ICRC said on Tuesday that the US had failed to reply to demands to provide a list of everyone it was holding.
“These people are, as far as we can tell, detained in locations that are undisclosed not only to us but also to the rest of the world,” spokeswoman Antonella Notari told The Associated Press.
The US Defense Department denied on Wednesday that it was hiding any imprisoned terrorism suspects from the ICRC, but a spokesman would not comment on whether other government agencies might be doing so.
"I can say that the ICRC has access to all detainee operations under our control," said Larry Di Rita, spokesman for Defense Secretary Donald Rumsfeld.
"And beyond that, I'm just not prepared to discuss it."
The Central Intelligence Agency (CIA) has declined to comment on whether it may be holding terrorism suspects at foreign locations.
Under the Geneva Conventions, which outline rules concerning the treatment of prisoners of war, the US is obliged to give the neutral ICRC access to PoWs and other detainees to check on their conditions and allow them to send messages to their families.
The US says it is cooperating with the agency and has allowed ICRC delegates access to thousands of prisoners in Afghanistan, the US naval base at Guantanamo Bay, Cuba, and in Iraq, where delegates have visited former Iraqi dictator Saddam Hussein.
It is unclear whether the Geneva Conventions cover terror suspects, but Notari said that “for humanitarian reasons” the ICRC should be told about all detainees.
ICRC President Jakob Kellenberger made the request in January on a visit to Washington during which he met Secretary of State Colin Powell, Deputy Defence Secretary Paul Wolfowitz and National Security Adviser Condoleezza Rice.
“So far we haven't had a satisfactory reply,” said Notari.
She said the US Federal Bureau of Investigation had posted details of arrested suspects on its website, but some of these people had never shown up in prisons visited by the ICRC.
Notari said she had read media reports that some people were being held at Diego Garcia, a British-held island in the Indian Ocean which is used as a strategic military base by the United States.
But she added that the ICRC had not been notified of any prisoners there.
“We just simply have absolutely no confirmation of this in any formal way,” she said.
In his report into allegations of abuse at Baghdad's Abu Ghraib prison, Major General Antonio Taguba found that military police there had “routinely held persons brought to them by other government agencies without accounting for them, knowing their identities, or even the reason for their detention”.
On at least one occasion they moved these “ghost detainees” around the prison to hide them from a visiting ICRC delegation, he added.
He described the actions as “deceptive, contrary to army doctrine, and in violation of international law”.
ICRC delegates visited nearly 500,000 detainees in around 80 countries last year, including almost 11,000 in Iraq.
swissinfo with agencies
The ICRC works mainly to protect the victims of conflict by providing humanitarian assistance and conducting PoW visits.
It monitors compliance of the Geneva Conventions, which outline the rules of law in times of war and occupation, including the treatment of PoWs.
Switzerland is the depository state of the Geneva Conventions.