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.