The Council of Europe has demanded an investigation into claims the US ran secret jails for terror suspects.
Suspicions have been raised about flights to a Polish airfield
The human rights watchdog called the claims "extremely worrying" and said such prisons would constitute a serious human rights violation.
A US newspaper said such prisons were set up in eight countries - some of them unnamed Eastern European states.
"It is essential that these allegations be thoroughly investigated," said Human Rights Commissioner Alvaro Gil-Robles.
"Such practices would constitute a serious human rights violation, and further proof of the crisis of values that the use of certain methods in the fight against terrorism is proving," he was quoted by the AFP news agency as saying.
The European Commission, meanwhile, says it will encourage states in eastern Europe to comment on the claims.
Some governments have already issued denials, including Romania and Poland which were named by New York-based Human Rights Watch (HRW) as possible hosts for the prisons.
The Commission will be seeking statements from countries that have not yet denied the existence of such prisons on their territories, said EU spokesman Friso Roscam Abbing.
HRW said on Thursday that a study of international air flight data, covering the summer of 2003, appeared to suggest the "black sites" in Europe were in Romania and a former military airport in north-east Poland.
Polish officials and border guards at Szczytno-Szymany airport confirm a Boeing carrying seven people with US passports landed on the date stated by HRW, the AFP news agency reports.
But the authorities - including the airport's former director - deny any knowledge of prisoner transfers and say they did not know where the plane came from or where it went to, AFP says.
Poland's outgoing Defence Minister Jerzy Szmajdzinski said: "We aren't detaining terrorists, or interrogating them, or doing anything else with them."
The denial theoretically leaves open the possibility that prisoners have been detained on Polish soil by the Americans themselves, says the BBC's Jan Repa.
Romanian Prime Minister Calin Tariceanu said: "There are no CIA bases in Romania."
Major terror suspects
The Washington Post reported on Wednesday that the centres - known as "black sites" - were set up in the wake of the 11 September attacks on the US in 2001.
About 30 detainees, considered major terrorism suspects, were held by the CIA at these sites, according to the paper.
Another group of at least 70 detainees had since been handed over to intelligence services in Egypt, Jordan, Morocco, Afghanistan and other countries.
All the sites had now been closed, the paper said.
The CIA has refused to comment.
Those with close links to the intelligence agencies say the US government keeps suspected al-Qaeda operatives secretly incarcerated on foreign soil so that they are not able to contest their detention in US courts and can be interrogated over a long period.