The United States has violated Pakistan's sovereignty and destroyed tribal structures with unmanned aerial drone strikes in its counterterrorism near the Afghan border, a UN human rights investigator has said.

"As a matter of international law, the US drone campaign in Pakistan is ... being conducted without the consent of the elected representatives of the people, or the legitimate Government of the State," Ben Emmerson, the UN's special rapporteur on human rights and counterterrorism, said in a statement issued by the Office of the High Commissioner for Human Rights in Geneva on Friday.

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.

"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 that 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 UN 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," Victoria Nuland, the US State department's spokesperson, said. "We have a strong ongoing counterterrorism dialogue with Pakistan and that will continue."

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

"We have a solid working relationship with [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 interference'

Emmerson said the Pashtun tribes of northwestern Pakistan's Federally Administered Tribal Areas (FATA), the largely lawless region bordering Afghanistan in Pakistan's west, 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 neighbouring Afghanistan.

Clearing out border sanctuaries used by forces fighting ISAF forces in Afghanistan is seen by Washington as crucial to bringing stability to Afghanistan, particularly as the US-led combat mission ends in 2014.

Civilian casualties

Most, but not all, attacks with unmanned aerial vehicles worldwide have been carried out 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 UN 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 centres 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.

Stanley McChrystal, the retired US general who devised the Pentagon'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.