Two United Nations human rights investigators have called for more transparency from the United States and other countries about their drone strikes programme, saying their secrecy is the biggest obstacle to determining the effect on civilian casualties.
Ben Emmerson and Christof Heyns, who presented two reports on the subject at the UN, also called on other countries to speak up about when deadly drone strikes are acceptable. They said the lack of consensus risked creating anarchy as more countries acquired the technology.
Mr Emmerson, the UN special rapporteur on human rights and counter-terrorism, said the US justified some drone strikes against terrorist targets in other countries by arguing that it was engaged in an armed conflict with al Qaida with no boundaries. He said other countries disagreed with that analysis but few had spelled out their own positions.
"We all recognise that the moment other states start to use this technology in similar ways, we are facing a situation which could escalate into a breakdown of peace and security," he said.
In his report, Mr Emmerson said he received statistics from the Pakistani government indicating that at least 2,200 people had been killed in drone strikes in that country since 2004. Of those, at least 400 were civilians.
But he said independent verification was difficult and the involvement of the CIA in counter-terrorism operations in both Pakistan and Yemen "has created an almost insurmountable obstacle to transparency".
Mr Emmerson said that any time civilians were killed "the state responsible is under an obligation to conduct a prompt, independent and impartial fact-finding inquiry and to provide a detailed and public explanation".
The US considers its drone program in Pakistan to be a key weapon against insurgent groups that it says stages cross-border forays into neighbouring Afghanistan. But many Pakistanis believe the strikes kill large numbers of civilians, raising tensions between the two countries and complicating their cooperation in the fight against militants.
Mr Heyns, the UN special rapporteur on extra-judicial executions, expressed disappointment with the US response to reports this week by Human Rights Watch and Amnesty International questioning the legality of the drone strikes.
White House spokesman Jay Carney said the US "would strongly disagree" with any claims that it acted improperly, arguing that American actions followed all applicable law. He said there must be "near-certainty" of no civilian casualties before the US proceeds with a drone strike. He said they were not used when targets could instead be captured.
Both Mr Emmerson and Mr Heyns said that the use of drone technology in a deadly strike is not the inherent problem. They said that in many cases, drone technology allows precision targeting that can reduce the number of civilian casualties.
"Drones are not inherently illegal weapons. They are here to stay," Mr Heyns said. "The main focus should be on legal parameters" on when to use them.
Mr Emmerson's report said the UN mission in Afghanistan had acknowledged drone strikes in that country - carried out by both the US and Britain - led to fewer civilian casualties than attacks using other weaponry.
In Yemen, his report said "the United States has largely succeeded in avoiding the infliction of large-scale loss of civilian life", with the exception of a cruise missile strike on a camp in 2009 that reportedly killed 40 civilians.
The strikes in Yemen are part of a joint US-Yemeni campaign against al Qaida in the Arabian Peninsula, called the most dangerous al Qaida branch.