Massoud Hossaini / AFP - Getty Images

In this file picture taken on on June 13, 2010, a U.S. Predator unmanned drone armed with a missile stands on the tarmac of Afghanistan's Kandahar military airport.

UNITED NATIONS -- The United States has violated Pakistan's sovereignty and shattered tribal structures with unmanned drone strikes in its counterterrorism operations near the Afghan border, a U.N. human rights investigator said in a statement on Friday.

U.N. special rapporteur on human rights and counterterrorism, Ben Emmerson, visited Pakistan for three days this week as part of his investigation into the civilian impact of the use of drones and other forms of targeted killings.

"As a matter of international law, the U.S. drone campaign in Pakistan is ... being conducted without the consent of the elected representatives of the people, or the legitimate Government of the State," Emmerson said in a statement issued by the Office of the High Commissioner for Human Rights in Geneva.

"It involves the use of force on the territory of another state without its consent and is therefore a violation of Pakistan's sovereignty," he said.

Emmerson said in January he would investigate 25 drone strikes in Pakistan, Yemen, Somalia, Afghanistan and the Palestinian territories. He is expected to present his final report to the U.N. General Assembly in October.

Washington had little to say about Emmerson's statement.

"We've seen his press release. I'm obviously not going to speak about classified information here," State Department spokeswoman Victoria Nuland said. "We have a strong ongoing counterterrorism dialogue with Pakistan and that will continue."

Spokesman Josh Earnest said the White House would withhold judgment until it sees Emmerson's full report.

"We have a solid working relationship with them (Pakistan) on a range of issues, including a close cooperative security relationship, and we're in touch with them on a regular basis on those issues."

'End military interference'
Emmerson said the Pashtun tribes of northwestern Pakistan's Federally Administered Tribal Areas, or FATA, Pakistan's largely lawless region bordering Afghanistan, have been decimated by the counterterrorism operations.

"These proud and independent people have been self-governing for generations, and have a rich tribal history that has been too little understood in the West," he said. "Their tribal structures have been broken down by the military campaign in FATA and by the use of drones in particular."

The tribal areas have never been fully integrated into Pakistan's administrative, economic or judicial system. They are dominated by ethnic Pashtun tribes, some of which have sheltered and supported militants over decades of conflict in neighboring Afghanistan.

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Clearing out militant border sanctuaries is seen by Washington as crucial to bringing stability to Afghanistan, particularly as the U.S.-led combat mission ends in 2014.

Most, but not all, attacks with unmanned aerial vehicles have been by the United States. Britain and Israel have also used them, and dozens of other countries are believed to possess the technology.

"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," Emmerson said.

The U.N. Human Rights Council asked Emmerson to start an investigation of the drone attacks following requests by countries including Pakistan, Russia and China.

Criticism of drone strikes centers on the number of civilians killed and the fact that they are launched across sovereign states' borders so frequently, far more than conventional attacks by piloted aircraft.

Retired U.S. General Stanley McChrystal, who devised the U.S. counterinsurgency strategy in Afghanistan, warned in January against overusing drones, which have provoked angry demonstrations in Pakistan.

Civilian casualties from drone strikes have angered local populations and created tension between the United States and Pakistan and Afghanistan. Washington has sought to portray civilian casualties as minimal, but groups collecting data on these attacks say they have killed hundreds of civilians.

Tabassum Zakaria and Roberta Rampton contributed to this report.