The CIA is running a network of secret prison facilities around the world to hold high-profile terror suspects, according to a US newspaper report.
The CIA has declined to comment on claims of a covert prison network
Such prisons are, or have been, located in Eastern Europe, Afghanistan and Thailand, the Washington Post claims.
It says more than 100 people have been sent to the facilities, known as "black sites", since they were set up in the wake of the 11 September attacks.
An intelligence agency spokesman told the BBC the CIA declined to comment.
Questioned about the report, White House spokesman Scott McClellan said: "I am not going into discussing any specific intelligence activities.
"I would say that the president's most important responsibility is to protect the American people. It's a responsibility he takes very seriously."
A Thai government spokesman denied playing host to secret detention facilities.
The Washington Post quotes current and former intelligence officers as saying that some top terror suspects are being held in an Eastern European country in a compound dating from Soviet times.
Its report says the covert prison system, financed by the CIA, has operated at various times in eight countries, as well as at the US naval base in Guantanamo Bay, Cuba.
"The hidden global internment network is a central element in the CIA's unconventional war on terrorism," the paper says.
Details of the system "are known to only a handful of officials in the United States and, usually, only to the president and a few top intelligence officers in each host country".
Almost nothing is known about who is held, the interrogation methods used, or how decisions are made about how long they are detained.
The names of the Eastern European countries allegedly involved were withheld at the request of senior US officials, the Post says, for fear their disclosure could put operations at risk.
The whereabouts of high-profile terror suspects is a closely guarded secret in Washington, says the BBC's Pentagon correspondent Adam Brookes.
The fate of such men as 11 September suspect Khalid Sheikh Mohammed is simply a mystery, our correspondent says, but there has long been an assumption that they are held in secret facilities outside the US other than Guantanamo Bay.
Individuals with close links to the intelligence agencies say the US government sees a compelling case for keeping suspected al-Qaeda operatives incarcerated secretly on foreign soil.
That way the suspects are not able to contest their detention in American courts and can be interrogated over a long period, our correspondent says.
The US has in the past faced questions over its use of "rendition", a process by which terror suspects are sent for interrogation by security officials in other countries, some of which are accused of using torture.
In August, human rights group Amnesty International called on the US to reveal details of its alleged secret detention of suspects abroad.
The group highlighted the case of two Yemeni men who claimed they were held in secret, underground US jails for more than 18 months without being charged.
During that time, they say, they were tortured for four days by the Jordanian intelligence services.