Washington (AFP) - America's use of drones to kill suspected jihadists around the world is driving hatred toward the United States and causing further radicalization, four former airmen have said.

In an open letter to President Barack Obama, Defense Secretary Ashton Carter and CIA Director John Brennan, the four former drone operators said they were involved in the killing of innocent civilians, and had gone on to suffer Post Traumatic Stress Disorder.

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

"This administration and its predecessors have built a drone program that is one of the most devastating driving forces for terrorism and destabilization around the world," they added.

The four are Brandon Bryant, Cian Westmoreland, Stephen Lewis and Michael Haas. Westmoreland was a transmissions expert and the other three controlled powerful sensors on Predator drones.

According to The Guardian, which published interviews with the men on Thursday, the four had 20 years drone operating experience between them.

They told the newspaper that drone operators quickly grow numb to their work and sometimes killed people even if they were unsure whether they were hostile or not.

In one case, Bryant said his drone team killed five tribal men and a camel traveling from Pakistan to Afghanistan, even though they weren't certain who they were or what they were doing.

"We waited for those men to settle down in their beds and then we killed them in their sleep," Bryant told the newspaper. "That was cowardly murder."

When he left the service, Bryant was given an envelope containing a report card with the number of killings he'd been involved in -- that number was 1,626.

Since taking office in 2009, Obama has vastly expanded the drone program, authorizing many more strikes than his Republican predecessor, George W. Bush.

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Several countries across the Middle East and Central Asia have seen deadly drone strikes.

According to whistleblower papers published by The Intercept website last month, the Obama administration has underrepresented the true number of civilians killed in drone strikes.

In classified slides, the US military describes fatalities from targeted strikes as "enemy killed in action," even if their identity is unknown or they were not the intended targets, according to The Intercept.

In one five month period, nearly 90 percent of those killed in airstrikes were not the intended targets, The Intercept said.

"We witnessed gross waste, mismanagement, abuses of power, and our country's leaders lying publicly about the effectiveness of the drone program," the men said in the letter.

"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."