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."

Emmerson's statement is carefully worded. He portrays himself as conveying Pakistan's concerns, rather than vouching for their particulars. But it's still the strongest statement yet by an international official calling for an end to a campaign of targeted killing that briefly flared back up earlier this year. And to call the strikes an unwarranted violation of Pakistan's sovereignty is tantamount to saying the U.S. is waging a war of aggression.

"The Pashtun tribes of the FATA area have suffered enormously under the drone campaign," Emmerson's statement continues, referring to the tribal areas. "It is time for the international community to heed the concerns of Pakistan, and give the next democratically elected government of Pakistan the space, support and assistance it needs to deliver a lasting peace on its own territory without forcible military interference by other States."

If the drone strikes continue into the next Pakistani government, Emmerson warned, the U.S. drone effort could further destabilize the nuclear power, undermining a key U.S. strategic goal at the heart of the drone strikes. He urged patience with a Pakistani military effort to eradicate al-Qaida's allies in the tribal areas – one that official Washington has long since written off as unserious.

Significantly and subtly, Emmerson raised doubts over repeated U.S. claims that the targeting efforts behind the drones kill terrorists and spare civilians. Last month, Sen. Dianne Feinstein, the chairwoman of the Senate Select Committee on Intelligence and a staunch drone advocate, claimed that the drones kill only "single digits" worth of civilians annually. Many of the CIA's strikes, termed "signature strikes," kill people believed to fit a pattern of extremist behavior, rather than killing specific, known terrorists.

Emmerson's tribal contacts gave reason to doubt that westerners unfamiliar with the area would even be able to tell a terrorist from an average resident.

"In discussions with the delegation of tribal Maliks from North Waziristan the Special Rapporteur was informed that drone strikes routinely inflicted civilian casualties, and that groups of adult males carrying out ordinary daily tasks were frequently the victims of such strikes," Emmerson continued. "They emphasized that to an outsider unfamiliar with Pashtun tribal customs there was a very real risk of misidentification of targets since all Pashtun tribesmen tended to have similar appearance to members of the Pakistan Taliban, including similar (and often indistinguishable) tribal clothing, and since it had long been a tradition among the Pashtun tribes that all adult males would carry a gun at all times. They considered that civilian casualties were a commonplace occurrence and that the threat of such strikes instilled fear in the entire community."

As much as Emmerson will rely on the Obama administration for access concerning the drones during his inquiry, he's given a major international platform to the victims and the critics of its robotic campaign. Emmerson told Danger Room last month that he endorsed John Brennan to run the CIA out of confidence that Brennan will rein in the drone effort. Now that Brennan's at Langley, Emmerson will soon have his theory tested, having delivered a major public challenge to Washington.