Missile strikes by US drones have often resulted in civilian deaths
The US has been warned that its use of drones 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."
Mr Alston raised the issue in a report to the UN General Assembly's human rights committee on Tuesday.
At a news conference afterwards, he said he had become increasingly concerned at the increase in their use since June, particularly in Afghanistan and Pakistan.
The US told the UN in June that it has a legal framework to respond to unlawful killings. It also said the UN Human Rights Council and the General Assembly have no role in relation to killings during an armed conflict.
But Mr Alston described that response as "simply untenable".
Mr Alston's warning came as US President Barack Obama reviews US strategy in the Afghan campaign.
The senior US military commander in Afghanistan, Gen Stanley McChrystal, has asked for at least 40,000 more troops there.