The UN human rights chief on Thursday called for a UN investigation into US drone strikes in Pakistan, questioning their legality and saying they kill innocent civilians.

UN High Commissioner for Human Rights Navi Pillay made the remarks at the end of a four-day visit to the country, where US drone strikes have on average targeted Islamist militants once every four days under US President Barack Obama.

Islamabad is understood to have approved the strikes on Al-Qaeda and Taliban targets in the past. But the government has become increasingly energetic in its public opposition as relations with Washington have nosedived.

"Drone attacks do raise serious questions about compliance with international law," Pillay told a news conference in Islamabad.

"The principle of distinction and proportionality and ensuring accountability for any failure to comply with international law is also difficult when drone attacks are conducted outside the military chain of command and beyond effective and transparent mechanisms of civilian or military control," she said.

She said the attacks violate human rights.

"I see the indiscriminate killings and injuries of civilians in any circumstances as human rights violations."

The UN human rights chief provided no statistics but called for an investigation into civilian casualties, which she said were difficult to track.

"Because these attacks are indiscriminate it is very, very difficult to track the numbers of people who have been killed," she said.

"I suggested to the government that they invite the United Nations Special Rapporteur on Summary or Arbitrary Executions and he will be able to investigate some of the incidents."

She said UN chief Ban Ki-moon had urged states to be "more transparent" about circumstances in which drones are used and take necessary precautions to ensure that the attacks involving drones comply with applicable international law.

"So therefore I stress the importance of investigating such cases and ensuring compensation and redress to the victims."

Washington releases few details about its covert drone programme in Pakistan but on Wednesday US Defense Secretary Leon Panetta described them as self-defence and promised that they would continue to target Al-Qaeda in Pakistan.