Navi Pillay was speaking at the end of a fact-finding visit to Pakistan.
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 ourselves". Drone attacks have become a central part of US counter-terror operations but Ms Pillay said they were legally problematic.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". More
