The European commission is to investigate claims the CIA is holding al-Qaida captives at Soviet era compounds in eastern Europe.

The detention centres are part of a global internment network that includes Guantánamo Bay, Cuba and the Bagram air base in Afghanistan, according to the Washington Post.

The facilities - referred to as "black sites" in classified White House and CIA documents - allow the US agency to hold terror suspects for as long as it likes, but virtually nothing is known about who is kept in them.

Poland and Romania are thought the most likely locations in Europe, according to the New York-based Human Rights Watch and Polish press reports.

If the reports are true, the secret jails would violate European human rights law prohibiting unlawful detention.

A commission spokesman said it would informally question the 25 national governments on the claims. "We have to find out what is exactly happening. We have all heard about this, then we have to see if it is confirmed," he said.

Hungary, Slovakia and Bulgaria have denied involvement. The Czech interior minister, Frantiszek Bublan, said the US had approached Prague to build a camp but the request was turned down.

Bulgaria and Romania are scheduled to join the European Union in 2007 and are compelled to sign up to EU human rights standards. Eight other former Soviet bloc nations, including the Czech Republic and Poland, became members in May 2004.

Eastern European Nato members have been some of Washington's staunchest allies in the "war on terror" and in Iraq.

The Washington Post said it knew the names of the European countries involved but was withholding them at the request of US officials, who argued disclosure could act as a spur to terrorist reprisals.

The exact location of the facilities is known only to a handful of officials in the US and the host countries. The internment network as a whole has been kept almost entirely secret from the US Congress, which is charged with overseeing the CIA's covert actions. The report said the CIA was holding the top 30 al-Qaida suspects at the secret facilities, where they were kept in dark cells, sometimes underground, in isolation from the outside world. They have no recognised legal rights, and no one outside the CIA is allowed to talk to them or see them. The covert prison system was set up nearly four years ago in eight countries, including a facility in Thailand that was closed down after its existence was made public in 2003. Concerns over the CIA's handling of prisoners escalated last week after it emerged that the US vice-president, Dick Cheney, and the agency's director, Porter Goss, asked Congress to exempt the agency from legislation banning the cruel and degrading treatment of prisoners. Sources told the Washington Post that the process has caused considerable internal debate within the CIA, where there is concern about the legality, morality and practicality of the system.