A UN investigator has called on the Obama administration to explain under what legal framework its drone war is justified and suggested that “war crimes” may have already been committed.

Christof Heyns, UN special rapporteur on extrajudicial, summary or arbitrary executions, urged Washington to clarify the basis under international law of the policy, in a report issued to the United Nations Human Rights Council.

“The (US) government should clarify the procedures in place to ensure that any targeted killing complies with international humanitarian law and human rights and indicate the measures or strategies applied to prevent casualties, as well as the measures in place to provide prompt, thorough, effective and independent public investigation of alleged violations,” he said.

Citing reports that the US has conducted follow-up drone strikes aimed at people coming to the strike scene to rescue the injured, Heyns said, if it is true, “those further attacks are a war crime.”

“Current targeting practices weaken the rule of law,” he said. “Killings may be lawful in an armed conflict [such as Afghanistan] but many targeted killings take place far from areas where it’s recognized as being an armed conflict.”

Heyns is not the first UN official to question the Obama administration’s drone war. The UN human rights chief Navi Pillay earlier this month called for a UN investigation into US drone strikes in Pakistan, arguing their questionable legality and that they indiscriminately kill innocent civilians.

“I see the indiscriminate killings and injuries of civilians in any circumstances as human rights violations,” she said, adding that, “Because these attacks are indiscriminate it is very, very difficult to track the numbers of people who have been killed.”

Obama administration officials told the New York Times recently that they “[count]all military-age males in a strike zone as combatants…unless there is explicit intelligence posthumously proving them innocent.” Operating on the principle of guilty until proven innocent is an extreme form of Executive Branch overreach.

Citing figures from the Pakistan Human Rights Commission, Heyns said US drone strikes killed at least 957 people in Pakistan in 2010 alone and that thousands total have been killed in over 300 drone strikes there since 2004. The Obama administration has only named a fraction of those killed.

Heyns questioned the administration’s “decisions to kill rather than capture ‘human targets,'” and said the legality of the drone strikes is undermined if “the State in which the killing takes place” has not given consent.

Pakistani Ambassador Zamir Akram also spoke to say that his country consistently maintained that the use of drones was illegal and violated the sovereignty of Pakistan.

“Thousands of innocent people, including women and children, have been murdered in these indiscriminate attacks,” Ambassador Akram said.

Twenty-six members of Congress have also called on the President to provide a legal justification for so-called “signature” drone strikes, which allow individuals to be targeted even when their identities are not known. So far, the administration has been silent in the face of these legal challenges.