The legality of drone attacks conducted by the United States in Pakistan have been questioned by United Nations Commissioner for Human Rights Navi Pillay.
Drone attacks are a central part of American counter-terror operations but after a fact-finding mission to Pakistan, Ms Pillay said they were legally problematic.
US officials defended the policy after al-Qaeda leader Abu Yahya al-Libi was killed in a drone strike in the North Waziristan tribal area at dawn on Monday.
Defence Secretary Leon Panetta said that the US would "continue to defend ourselves".
The policy has contributed to a recent worsening in relations between the US and Pakistan, the BBC reports.
The Pakistani foreign ministry summoned the American deputy ambassador in Islamabad on Tuesday to protest at recent drone attacks.
"Drone attacks do raise serious questions about compliance with international law, in particular the principle of distinction and proportionality," Ms Pillay said.
"Ensuring accountability for any failure to comply with international law is also difficult when drone attacks are conducted outside the military chain of command."
Navi Pillay also voiced concerns that the strikes were being conducted "beyond effective and transparent mechanisms of civilian or military control".
Ms Pillay said she has suggested to the Pakistani government that they invite the United Nations Special Rapporteur on Summary or Arbitrary Executions to investigate some of the incidents.