A UN human rights expert on Wednesday urged the United States to sideline the CIA from targeted killings using drones, warning that the practice amounted to "a licence to kill without accountability".

In a report to the UN Human Rights Council, Philip Alston, the special rapporteur on extrajudicial executions, warned that the "prolific" US use of targeted killings, mainly by unmanned aircraft, was setting a damaging example that other countries would follow.

"I'm particularly concerned that the United States seems oblivious to this fact when it asserts an ever-expanding entitlement for itself to target individuals across the globe," he told the 47-member council.

"But this strongly asserted but ill-defined licence to kill without accountability is not an entitlement which the United States or other states can have without doing grave damage to the rules designed to protect the right to life and prevent extrajudicial executions."

Alston's study on targeted killings sharply criticised the legal arguments invoked to justify them, their civilian toll and the involvement of the Central Intelligence Agency (CIA).

"Intelligence agencies, which by definition are determined to remain unaccountable except to their own paymasters, have no place in running programmes that kill people in other countries," Alston told the rights council.

Countries had to demonstrate that they were complying with rules limiting killings of targeted individuals to those directly involved in fighting, he underlined.

"The clearest challenge to this principle today comes from the programme operated by the US Central Intelligence Agency in which targeted killings are carried out from unmanned aerial vehicles or drones," Alston said.

He warned that hundreds of people had been killed including innocent civilians yet the CIA criteria for targeted killings remained shrouded in official secrecy.

"In a situation in which there is no disclosure of who has been killed, for what reason, and whether innocent civilians have died, the legal principle of international accountability is, by definition, comprehensively violated," he added.

The United States is conducting drone attacks in Afghanistan and in a covert manner in Pakistan's northwestern tribal belt, where officials say Islamist extremists hatch attacks on troops fighting in Afghanistan and on cities abroad.

Alston contrasted CIA practice with the US military, praising an army investigation that this week blamed human error for the death of 23 civilians in a drone-borne missile attack in Afghanistan in February.

"While it is by no means perfect, the US military has a relatively public accountability process," he said.

The incident sparked widespread anger at the presence of international troops in Afghanistan, and an apology from the commander of NATO forces in the country.