Until recently, the Pentagon has kept quiet on the amount of Air Force drone crashes. In the past year alone, at least $2 million in damaged drones occurred. More importantly, the drone losses are impacting the resources that field commanders have to work with in many of the countries where counter terrorism efforts are taking place.

A record number of Air Force drones crashed in major accidents last year, documents show, straining the U.S. military’s fleet of robotic aircraft when it is in more demand than ever for counter terrorism missions in an expanding array of war zones.

Driving the increase was a mysterious surge in mishaps involving the Air Force’s newest and most advanced “hunter-killer” drone, the Reaper, which has become the Pentagon’s favored weapon for conducting surveillance and airstrikes against the Islamic State, al-Qaeda and other militant groups.

The Reaper has been bedeviled by a rash of sudden electrical failures that have caused the 21/2-ton drone to lose power and drop from the sky, according to accident-investigation documents obtained under the Freedom of Information Act. Investigators have traced the problem to a faulty starter-generator, but have been unable to pinpoint why it goes haywire or devise a permanent fix.

All told, 20 large Air Force drones were destroyed or sustained at least $2 million in damage in accidents last year, the worst annual toll ever, according to a Washington Post investigation. The Pentagon has shrouded the extent of the problem and kept details of most of the crashes a secret.

The aircraft losses pose another challenge for the Air Force as it struggles to provide sufficient drone coverage for counter terrorism operations in Iraq, Syria, Afghanistan, Somalia, Yemen, Libya, Mali and Cameroon, among other countries.

Despite a surge in requests from field commanders, the Air Force last year had to curtail its drone combat missions by 8 percent because of an acute shortage of pilots for the remote-controlled aircraft. Things have gotten so bad that the Air Force is offering retention bonuses of up to $125,000 to its drone pilots, who have long complained of overwork.

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The Air Force also has contracted out more drone missions to private companies to meet what one general called “a virtually insatiable appetite” from military commanders for airborne surveillance.

While Air Force leaders have publicly bemoaned a lack of personnel and resources, they have said little about the high number of drone crashes, a long-standing vulnerability that worsened substantially last year.

Ten Reapers were badly damaged or destroyed in 2015, at least twice as many as in any previous year, according to Air Force safety data.

The Reaper’s mishap rate — the number of major crashes per 100,000 hours flown — more than doubled compared with 2014. The aircraft, when fully equipped, cost about $14 million each to replace.

The Air Force’s other primary drone model, the Predator, also suffered heavy casualties.

An older and less capable version of the Reaper, the Predator was involved in 10 major accidents last year. That’s the most since 2011, when the U.S. military was simultaneously surging troops into Afghanistan and withdrawing ground forces from Iraq.

Although the Defense Department has a policy to disclose all major aircraft mishaps, it did not publicly report half of the 20 Reaper and Predator accidents last year.

In five other cases, U.S. military officials provided confirmation only after local authorities reported the crashes or enemy fighters posted photos of the wreckage on social media.

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According to the military, only one drone was downed by hostile forces: a Predator that was hit by Syrian air defenses near Latakia on March 17.

All but one of the 20 Air Force drone accidents last year occurred overseas. Six drones crashed in Afghanistan. Four crashed in the Horn of Africa, near a U.S. military base in Djibouti. Three crashed in Iraq. There were also crashes in Kuwait, Turkey, Syria and Libya.

In two cases, Air Force officials would not identify the country where the mishaps occurred.