GENEVA - UN human rights experts warned on Wednesday that "widespread and systematic" secret detention of terror suspects could pave the way for charges of crimes against humanity.
In their first in-depth global study on the practice, the experts said the practice had spread to almost all regions of the world and was continuing.
The study, which is due to be submitted to the UN Human Rights Council in March, listed 66 states that have been involved in secret detentions, mainly over the past nine years.
In spite of international norms protecting individual rights, "secret detention continues to be used in the name of countering terrorism around the world," the report added.
"If resorted to in a widespread and systematic manner, secret detention might reach the threshold of a crime against humanity," the authors cautioned in their executive summary.
The "global war on terror", which was launched by President George W. Bush's administration after the September 11 attacks, had "reinvigorated" the use of secret detentions in an organised manner, the authors of the study said.
The campaign marked "the progressive and determined elaboration of a comprehensive and coordinated system of secret detention of persons suspected of terrorism, involving not only US authorities, but also other states in almost all regions of the world," it added.
The study was compiled by two independent UN experts on counter-terrorism and torture, as well as UN panels overseeing arbitrary detention and enforced disappearances.
While commitments by President Barack Obama to dismantle and investigate secret detentions were welcomed, the experts also called for clarification of grey areas including short term CIA holding facilities and those operated by the military Joint Special Operation Command.
Human rights campaigners believe suspects were detained in secret interrogation centres or prisons in recent years, and sometimes tortured or ill treated there, while other countries cracked down on political opponents or restive ethnic groups.
Extraordinary rendition involved abducting suspects without legal proceedings, and flying them to foreign countries or secret CIA prisons.
Drawing on its own interviews with former detainees, witnesses, officials and its own analysis of flight records, as well as published material, the UN study named dozens of secret detainees -- including some alleged to have died in custody.
Thailand denied in a submission to the UN experts that it had hosted a secret detention facility for the United States, according to the study.
Nonethless, the experts maintained that it was "credible that a CIA black site" existed in Thailand, and called on Thai authorities to launch an independent investigation.
The study also welcomed a Lithuanian parliamentary inquiry into similar allegations, which had concluded that there was no evidence to back them up.
However, it stressed that "the findings can in no way constitute the final word on Lithuania's role in the programme."
The UN study also cited evidence of secret US-run facilities in Romania, Poland, and Kosovo as well as several in Afghanistan and Iraq, including "Dark Prison" and "Salt Pit."
Consistent allegations by detainees "adds weight to claims that Jordan, Egypt, Morocco, Syria, Pakistan, Ethiopia and Djibouti" were also used as "proxy states to hold detainees on the CIA's behalf," the report added.
The UN study called on countries to promptly investigate allegations, noting that there had been virtually no judicial proceedings or prosecutions on such cases.