A United Nations human rights investigator says the use of drones by the United States against the Taliban and al Qaeda in Afghanistan and Pakistan may breach international law.
UN human rights investigator Philip Alston said the US should explain the legal basis for attacking individuals with 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 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.
The United States 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.
Mr Alston described that response as "simply untenable".
President Barack Obama is reviewing US strategy in the Afghan campaign. Senior US military commander in Afghanistan, General Stanley McChrystal, has asked for at least 40,000 more troops there.
