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.
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".