The report described secret prisons as a "horrendous" practise
The UN is to investigate the use of secret detention centres allegedly used by the CIA and other groups as part of US counter-terrorism efforts.
The decision follows a UN report which uncovered what it said were serious abuses by the intelligence services.
The report said secret prisons were "one of the most horrendous practises" adopted by intelligence services.
The UN experts carrying out the probe have urged all countries to cooperate fully with their investigation.
The UN's Special Rapporteur on Torture, Manfred Nowak, said there was "credible" evidence that the US had sent terror suspects to secret detention centres in the Middle East, Eastern Europe and Asia.
There was also strong evidence that other countries, including Britain, Canada, Kenya and Pakistan, had helped to secure the arrest and extraordinary rendition of such suspects, he said.
Martin Scheinin, UN special rapporteur on human rights and counter-terrorism, said that in many cases states receiving detainees had "reportedly engaged in torture and other forms of ill-treatment".
Mr Scheinin told the UN Human Rights Council in Geneva that he welcomed US President Barack Obama's decision to close the Guantanamo Bay detention centre.
But he said that in other countries "this practice or permission of secret detentions, often of people who have been branded as terrorist suspects, is continuing".
"Before a page can be turned, we have to know what's on it, in order to move forward," he said.
Both men urged all UN member states to cooperate with the year-long investigation.
They said this would not only enable them to clarify the facts, but also to ensure that such centres were not used in the future.