UNITED NATIONS (AP) -- Two U.N. human rights investigators called for more transparency from the United States and other countries about their drone strikes program, saying their secrecy is the biggest obstacle to determining the impact pact on civilian casualties.

Ben Emmerson and Christof Heyns, who presented two reports on the subject at the United Nations on Friday, also called on other countries to speak up about when deadly drone strikes are acceptable. They said the lack of consensus risks creating anarchy as more countries acquire the technology.

Emmerson said the U.S. has justified some drone strikes against terrorist targets in other countries by arguing that it is engaged in an armed conflict with al-Qaida with no boundaries. He said other countries disagree with that analysis but few have spelled out their own positions.

"We all recognize 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," said Emmerson, the U.N. special rapporteur on human rights and counter-terrorism.

In his report, Emmerson said he received statistics from the Pakistani government indicating that at least 2,200 people have been killed in drone strikes in that country since 2004. Of those, at least 400 were civilians. But Emmerson said independent verification is difficult and the involvement of the CIA in counter-terrorism operations in both Pakistan and Yemen "has created an almost insurmountable obstacle to transparency."

Emerson said that any time civilians are 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 U.S. considers its drone program in Pakistan to be a key weapon against insurgent groups that it says stages cross-border forays into neighboring 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.

Heyns, the U.N. special rapporteur on extrajudicial executions, expressed disappointment with the U.S. 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 U.S. "would strongly disagree" with any claims that the U.S. had acted improperly, arguing that American actions follow all applicable law. He said there must be "near-certainty" of no civilian casualties before the U.S. proceeds with a drone strike. He said they're not used when targets can instead be captured.

Both Emmerson and Heyns said that the use of drone technology in a deadly strike is not the inherit 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," Heyns said. "The main focus should be on legal parameters" on when to use them.

Emmerson's report said the U.N. mission in Afghanistan has acknowledged drone strikes in that country _ carried out by both the U.S. and Britain _ have 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 U.S.-Yemeni campaign against al-Qaida in the Arabian Peninsula, called the most dangerous al-Qaida branch.