The use of drones by U.S. intelligence agencies to target suspected militants in Afghanistan, Pakistan and elsewhere lacks the accountability required under international law, a U.N. human rights expert said Friday.

Philip Alston, a New York University law professor, will call next week for new international rules to govern the use of drones to ensure they are deployed in line with the laws of war.

The CIA's program of drone strikes against suspected al-Qaida and Taliban insurgents has never been publicly acknowledged by U.S. administration officials, even though it has been written about extensively in the media.

A CIA spokesman said last month that the intelligence agency's counterterror operations are conducted in strict accord with the law.

"In my view there is no legal prohibition on CIA agents, or you and me, deciding to take a 'direct part in hostilities,' which is not to say that it is desirable," Alston told The Associated Press in an e-mail Friday.

"The problem for me is that when this happens, especially as a matter of state policy, there is no willingness to comply with any of the requirements as to transparency and accountability which are central to international humanitarian law."

The independent U.N. investigator is due to present a report to the Geneva-based Human Rights Council on Thursday about the use of drones, or unmanned aerial vehicles, to carry out targeted killings.

In an interview Tuesday on Australia's ABC radio, Alston said only the United States, Israel and Russia currently use drones to carry out targeted killings, but other countries were likely to begin using them for similar purposes in future.

"We've got to look at rules for the future, which will govern all countries," he told ABC Radio.

Alston, an Australian, suggested the rules should specify that suspects who aren't wearing uniforms could only be targeted if they are directly observed taking part in hostilities.

More importantly, he said, countries needed to ensure that those charged with carrying out the drone strikes could be held accountable.

"We have to know who they are targeting. Not lists of names or anything like that, but the criteria that are being used, and then there's got to be some follow-up," Alston said in the radio interview.

"The CIA, by definition, is not accountable" except directly to President Barack Obama, he said.

Alston suggested that, unless the intelligence agency's work could be made transparent, the role of conducting drone strikes should be transferred to the military, who were better versed in — and capable of abiding by — international law.

Officials at the U.S. mission to the U.N. in Geneva didn't immediately respond to a request for comment.