Lethal U.S. drone strikes in the Middle East are fueling hatred toward the West, spurring the expansion of militant groups such as the Islamic State, and doing psychological harm to drone operators, according to a group of former U.S. military airmen.

Four former drone operators have written an open letter to President Barack Obama asking him to reevaluate his administration's use of drones, which critics say can kill innocent civilians and drive angry survivors into militancy.

"You harm these people, and they're going to want revenge," former Staff Sergeant Brandon Bryant, who operated drones for the U.S. Air Force Predator program between 2007 and 2011, said Thursday at a news conference in New York.