Allegations that the CIA abducted and illegally transported terror suspects across European borders are credible, an investigator has said.
Swiss senator Dick Marty has submitted a report on the claims, made in the media, to a meeting of the human rights committee of the Council of Europe.
Mr Marty criticised the US for refusing to confirm or deny the allegations.
The US government and its intelligence agencies say that all their operations are conducted within the law.
Mr Marty's findings were released in an official statement by a committee of the Council of Europe, the continent's human rights watchdog.
"The elements we have gathered so far tend to reinforce the credibility of the allegations concerning the transport and temporary detention of detainees - outside all judicial procedure - in European countries," he said.
Condoleezza Rice faced questions over the claims while in Europe
He went on: "Legal proceedings in progress in certain countries seemed to indicate that individuals had been abducted and transferred to other countries without respect for any legal standards."
The BBC's Alix Kroeger in Strasbourg says the strongly worded report will add to the pressure for more in-depth inquiries.
The European Union has so far declined to investigate, although it has said any member state with secret prisons on its territory could have its EU voting rights suspended.
Poland and Romania have been named by the media as possible locations of CIA secret prisons, but have denied the allegations.
Mr Marty said it was "still too early to assert that there had been any involvement or complicity of member states in illegal actions".
But, he warned, if the allegations proved correct any European states involved "would stand accused of having seriously breached their human rights obligations to the Council of Europe".
Mr Marty urged the US to comment formally on the allegations, saying he "deplore[d] the fact that no information or explanations" were given during last week's tour of Europe by US Secretary of State Condoleezza Rice.
Ms Rice refused to address claims the CIA operated secret prisons abroad, where suspects could be interrogated without reference to international law.
She said American interrogators were bound by a UN treaty banning the use of torture, regardless of whether they were working in the US or abroad.
A group of British MPs investigating the matter, the All Party Parliamentary Group on Extraordinary Rendition, said the UK could be at risk of breaching its legal obligations.
International law expert Professor James Crawford, of Cambridge University, told the group the UK government must satisfy itself on the issue of torture rather than relying on US assurances.
"A government is not exonerated from conduct which leads directly to a person being tortured merely by closing its eyes to that prospect," he said.