A US newspaper said such prisons were set up in eight countries - some of them unnamed Eastern European states.

A rights group has suggested Romania and Poland might have been involved, but both states have issued denials.

The International Committee of the Red Cross has said it wants access to all foreign terror suspects held by the US.

Chief spokeswoman Antonella Notari said it was concerned about the fate of an unknown number of people captured as part of the Bush administration's war on terror and allegedly held at undisclosed places of detention.

Human rights laws

The centres - known as "black sites" - were set up in the wake of the 11 September attacks on the US in 2001, says the Washington Post.

Those 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 US courts and can be interrogated over a long period, they say.

EU spokesman Friso Roscam Abbing told the BBC News website that its justice experts would be contacting European Union member states over the issue.

But he stressed that a formal investigation had not been launched.

Mr Roscam Abbing said that any such prisons would probably violate EU human rights laws.

"We have seen the reports and now we need to look into the issue and make contact with the appropriate authorities," he said.

"Experts from our Directorate-General Justice, Freedom and Security will make contact with those authorities."

Denials

According to the Washington Post, about 30 detainees, considered major terrorism suspects, were held by the CIA in the "black sites".

About 70 others have been delivered to intelligence services in countries like Egypt, Jordan and Morocco - some via the "black sites" - in a process known as "rendition" which was already public knowledge.

US-based Human Rights Watch has said that a study of international air flight data, covering the summer of 2003, appears to point to a location in Romania and a former military airport in north-east Poland.

The claims have prompted a flurry of denials.

Romanian Prime Minister Calin Tariceanu said: "There are no CIA bases in Romania".

Poland saw the swearing in of a new government on Monday. Former Defence Minister Jerzy Szmajdzinski said: "We aren't detaining terrorists, or interrogating them, or doing anything else with them."

The BBC's Jan Repa says the insistence by Polish officials that they have not detained prisoners at America's request theoretically leaves open the possibility that prisoners have been detained on Polish soil by the Americans themselves.

Meanwhile Czech Interior Minister Frantisek Bublan said the US had asked his country to take some of the prisoners being held by the Americans at the Guantanamo base who had so far not been charged with any crime.

He said the request had been rejected amid security fears, adding that 10 other countries had been approached and had rejected the request.

High-profile suspects

The Washington Post also named Afghanistan and Thailand as hosts of the secret jails, all of which are now said to have closed. Thailand has issued a denial.

The newspaper said it had not published the names of Eastern European countries involved in the programme at the request of senior US officials, who had argued that doing so could damage counter-terrorism efforts and lead to retaliation by terrorists.

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.

Commentary:
Under the Geneva Conventions the Red Cross has the right to talk to all POW's. Since they don't even know where they're at, this is another impeachable offense.