The US has been warned that its use of drones—also used recently against pirates—to target suspected terrorists in Afghanistan and Pakistan may violate international law.
UN human rights investigator Philip Alston said the US should explain the legal basis for attacking individuals with the remote-controlled aircraft.
He said the CIA had to show accountability to international laws which ban arbitrary executions.
Drones have killed about 600 people in north-west Pakistan since August 2008.
Mr Alston, the UN Special Rapporteur on Extrajudicial Executions, told the BBC: “My concern is that these drones, these Predators, are being operated in a framework which may well violate international humanitarian law and international human rights law.
“The onus is really on the government of the United States to reveal more about the ways in which it makes sure that arbitrary executions, extrajudicial executions, are not in fact being carried out through the use of these weapons.”