In Yemen, the group estimates, the United States may have carried out more than 100 drone strikes. Together with other U.S. operations, anywhere between 374 and 1,112 people, of whom 72 to 178 were civilians, were killed, the group estimates.
Obama told CNN that a target must meet "very tight and very strict standards," and Brennan said that in "exceedingly rare" cases, civilians have been "accidentally injured, or worse, killed in these strikes."
Do other countries use drones?
As CNN National Security Analyst Peter Bergen puts it, a decade ago the United States "had a virtual monopoly on drones. Not anymore."
More than 70 countries now have some type of drone -- although only a few possess armed drones, according to The New American Foundation.
Iran has claimed to have an armed drone of its own.
China unveiled 25 drone models in 2010, some of which were outfitted to fire missiles.
"Only the United States, United Kingdom and Israel are known to have launched drone strikes against their adversaries, although other members of the International Security Assistance Force in Afghanistan, such as Australia, have 'borrowed' drones from Israel for use in the war there," Bergen wrote in October.
For many years, Israel led the world in developing Unmanned Aerial Vehicle systems (UAS), according to the Congressional Research Service.
A 2011 study by Aerospace America found 680 UAS programs worldwide.
How else are drones used?
Drones are a rapidly growing form of technology, used for numerous purposes outside the military.
Some law enforcement agencies are using them. Days ago, the FBI used surveillance drones to monitor a hostage standoff involving a 5-year-old boy in Alabama.
The National Oceanic and Atmospheric Administration uses drones to study weather systems and ecosystems.
The Federal Aviation Administration has announced progress in helping integrate UAS into the U.S. aerospace system.
And numerous private companies have sprung up in the last few years to make small remote-controlled mini-aircraft moutned with cameras available for sale.
As CNN Money explains, "Journalists and sports photographers use them in lieu of expensive helicopters. Real estate agents employ them for aerial photos and video. Wildlife researchers and search-and-rescue outfits are using them or studying the potential. Even the utility industry is interested in having them hunt for downed power lines after a storm."