After days of meeting with Pakistani officials, the United Nations official investigating Washington’s global campaign of drone strikes attacked the legal and strategic basis for the robotic war in its biggest battlefield. And he raised doubts over whether Americans operating the drones can actually distinguish terrorists from average Pakistanis.

Ben Emmerson spent much of the week in Pakistan soliciting the views of senior government and elected officials about the drone strikes, part of his ongoing effort to investigate the relatively new method of targeted killing. He said in a statement on Friday that he also met with representatives of the tribal areas of western Pakistan that have borne the overwhelming brunt of the drone campaign. The officials underscored to Emmerson that Pakistan doesn’t consent to the U.S. drone effort, and denied extending the tacit consent that its military — with whom Emmerson did not consult — has previously provided.

“As a matter of international law the U.S. drone campaign in Pakistan is therefore being conducted without the consent of the elected representatives of the people, or the legitimate Government of the State,” Emmerson, the U.N. special rapporteur on human rights and counterterrorism, said in the statement. “It involves the use of force on the territory of another State without its consent and is therefore a violation of Pakistan’s sovereignty.”