Ben Emmerson, the U.N. special rapporteur on counter-terrorism and human rights, told a Harvard Law School audience he would create a special investigative unit in Geneva early next year to examine U.S. drone attacks' legality.
He said this summer some drone strikes in Pakistan might amount to war crimes.
The investigative unit will be set up through the intergovernmental U.N. Human Rights Council with South African Christof Heyns, the U.N. special rapporteur on extrajudicial, summary or arbitrary executions, Emmerson said.
"Rapporteur" is a French-derived word for an investigator who reports to a deliberative body.
The investigation will also look at "other forms of targeted killings conducted in counter-terrorism operations, in which it is alleged that civilian casualties have been inflicted," Emmerson said in his speech.
The investigation would also examine drone and other targeted assassinations by other governments that result in civilian deaths or injuries, he said.
Emmerson said his decision to investigate the combat-drone attacks and other targeted killings reflected his frustration with the Obama administration's unwillingness to release information about the strikes.
In doing so, "the administration is holding its finger in the dam of public accountability," Emmerson said.
The White House had no immediate comment on Emmerson's remarks. The U.S. mission to the United Nations said White House counter-terrorism adviser John O. Brennan previously expressed the administration's position.
Brennan defended the U.S. program April 30 as "legal, ethical and wise."
"Yes, in full accordance with the law -- and in order to prevent terrorist attacks on the United States and to save American lives -- the United States government conducts targeted strikes against specific al-Qaida terrorists, sometimes using remotely piloted aircraft, often referred to publicly as drones," he told the Woodrow Wilson International Center for Scholars in Washington.
Brennan said "the rigorous standards and process of review to which we hold ourselves" make civilian deaths or injuries "exceedingly rare."
If civilians are killed, "it pains us and we regret it deeply, as we do any time innocents are killed in war," Brennan said.
At Harvard, Emmerson touched on the U.S. presidential campaign, saying both President Barack Obama and Republican presidential nominee Mitt Romney agreed during Monday night's foreign-policy debate on the value of the covert use of combat drones.
He also said Obama's concepts and assumptions about fighting terrorism increasingly resembled the "global war paradigm" of President George W. Bush. He said Obama retreated from such a paradigm for the first 2 1/2 years he was in the White House.
The paradigm, which views the struggle against terrorism as a permanent war, "was always based on the flimsiest of reasoning," Emmerson said.
A secret global terrorist-targeting database known as a "disposition matrix," reported by The Washington Post Wednesday, is part of a shift in the administration's worldview about terrorism, officials told the newspaper.
The shift reflects an evolution in the administration's basic assumptions about fighting terrorism from finite emergency measures after the Sept. 11, 2001, terrorist attacks on the United States to institutionalized fixtures of the national security apparatus, the officials said.
U.S. Rep. Dennis Kucinich, D-Ohio, responded to the Post report saying, "state-sponsored murder has killed so many people that it is becoming routine, bureaucratized and pathological."
A Pakistani security official said Thursday at least three suspected Taliban-related terrorists were killed in a U.S. drone attack in Pakistan's mountainous North Waziristan region.
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