In an open letter to US President Barack Obama, Defense Secretary Ashton Carter and CIA Director John Brennan, the four former drone operators - Brandon Bryan, Cian Westmoreland, Stephen Lewis and Michael Haas - called for a reevaluation of the use of drones.

"We cannot sit silently by and witness tragedies like the attacks in Paris, knowing the devastating effects the drone program has overseas and at home."

Revenge on the West

The former airmen said they often inadvertently killed non-assailants and had the impression they were considered cowardly by relatives of victims. In turn, survivors often sought revenge on the West for what they considered pointless deaths, the group said.

"We came to the realization that the innocent civilians we were killing only fueled the feelings of hatred that ignited terrorism and groups like [Islamic State (IS)], while also serving as a fundamental recruitment tool," the men wrote.

"You harm these people, and they're going to want revenge," Bryant said at a news conference in New York.

Psychological trauma

After been involved in killing innocent civilians, the group, which had 20 years drone operating experience between them, also said they later suffered from Post Traumatic Stress Disorder (PTSD). Haas said many operators developed alcohol and drug addictions as a result of the psychological trauma.

"This isn't a videogame," Haas said. "When you fail and kill the wrong guy... you [can't] start over again."

The former senior airman, who trained other US pilots, said many operators also became desensitized to the drone strikes because of the distance from their targets.

As an example of the apathy developed by some drone operators, Haas said the term "fun-sized terrorists" would sometimes be used in reference to children in target areas.

'More civilian deaths than presented'

Obama has expanded the US drone program immensely since taking office in 2009 and has authorized many more strikes than his predecessor George W. Bush.

According to whistleblower papers published by "The Intercept" website in October, the number of civilian fatalities in drone strikes is much higher than figures presented by the White House.

ksb/bw (Reuters, AFP)