US military drone attacks in Pakistan raise serious legal questions, the UN's human rights chief has said.
Navi Pillay was speaking at the end of a fact-finding visit to Pakistan.
Drone attacks have become a central part of US counter-terror operations but Ms Pillay said they were legally problematic.
US officials defended the policy after al-Qaeda leader Abu Yahya al-Libi was reportedly killed in a drone strike earlier this week.
US Defence Secretary Leon Panetta said that the US would "continue to defend [itself]".
On Tuesday the Pakistani foreign ministry summoned the US deputy ambassador in Islamabad 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", she added.
Ms Pillay also voiced concerns that the strikes were being conducted "beyond effective and transparent mechanisms of civilian or military control".
Ms Pillay said she had called on Pakistan to invite the UN Special Rapporteur on Summary or Arbitrary Executions to investigate some of the incidents.
The US has also carried out drone strikes as part of military operations in Yemen, Somalia and Afghanistan.
Although an on-the-ground investigation in Pakistan by the Associated Press this year found that "the drone strikes were killing far fewer civilians than many Pakistanis are led to believe and that a significant majority of the dead were combatants", the policy is still deeply unpopular at local level.
This month, a major US newspaper said that drone strikes had replaced the US detention centre at Guantanamo Bay as the prime recruiting sergeant for al-Qaeda's cause.
The policy has also contributed to a recent worsening in relations between the US and Pakistan.
One controversial aspect of drone attacks in Pakistan is that they are not conducted by the US military - which is expected to comply with the laws of armed conflict - but by the Central Intelligence Agency, whose operations are far from transparent, the BBC's defence correspondent Jonathan Marcus reports.
The legality of the operations is also brought into question by the fact that Pakistan, unlike neighbouring Afghanistan, is not a zone of armed conflict, he adds.