And it was highly likely that European governments knew of it.
Swiss senator Dick Marty, however, said on Tuesday there was no formal evidence so far of the existence of clandestine detention centres in Romania or Poland as alleged by the New York-based Human Rights Watch.
"There is a great deal of coherent, convergent evidence pointing to the existence of a system of 'relocation' or 'outsourcing' of torture," Marty said in a report presented to the Council of Europe, the human rights watchdog on whose behalf he is investigating.
"Acts of torture or severe violation of detainees' dignity through the administration of inhuman or degrading treatment are carried outside national territory and beyond the authority of national intelligence services."
The report said that extraordinary rendition - transferring terror suspects to countries where they may face torture or ill treatment - "seems to have concerned more than a hundred persons in recent years.."
"It is highly unlikely that European governments, or at least their intelligence services, were unaware," it said.
"It is highly unlikely that European governments, or at least their intelligence services, were unaware"
Report presented to the
Council of Europe
The Council of Europe launched its inquiry after allegations surfaced in November that US agents interrogated main al-Qaida suspects at clandestine prisons in eastern Europe and transported some suspects to other countries passing through Europe.
Human Rights Watch identified Romania and Poland as possible sites of secret US-run detention facilities.
Both countries have denied involvement.
Rights treaties violated
Clandestine detention centres would violate European human rights treaties.
Marty's report said there was no formal, irrefutable evidence of the existence of secret CIA prisons in Romania, Poland or any other country.
"Nevertheless, there are many indications from various sources which must be considered reliable, justifying the continuation of ... investigative work," the report said.