Spy drones are getting shot down in air-to-air combat across the Middle East and South Asia.

Get used to it. Clashes between drones and manned warplanes could become much more frequent in coming years. And eventually, the drones might start winning.

On June 8, a US Air Force F-15E manned fighter jet intercepted and shot down an Iranian-made Shahed-129 armed drone that had just attacked US-allied fighters in southern Syria. It was unclear who—Iran, the Syrian regime, or some pro-regime, Iran-backed militia—was operating the Shahed drone.

Two weeks later, on June 20, an American F-15E destroyed another armed Shahed-129 as it flew toward pro-US fighters in the same area where the previous Shahed had gone down. The very same day, a Pakistani JF-17 manned fighter destroyed an unspecified Iranian drone as it reportedly flew on Pakistan's side of its border with Iran.

"Current military UAVs, including those in the US arsenal and the Iranian Shahed that was recently shot down, look like sitting ducks to a manned fighter aircraft."

If it seems like drone shoot-downs are becoming more common, it's because they are. No sooner had armed forces across the world begun adopting large, remote-controlled aircraft in the mid-1990s than enemy aircraft began trying to shoot them down. But the engagements were rare.

In the spring of 1999, a Serbian Mi-8 transport helicopter flew alongside a US Air Force Predator drone surveilling Kosovo during the NATO intervention in that country. The door gunner on the Serbian copter opened fire, destroying the American drone.

Three years later, in December 2002, an Iraqi MiG-25 fighter intercepted a US Predator drone spying on southern Iraq. The Americans had outfitted the Predator with air-to-air missiles for self-defense, but the MiG-25 was faster and its weapons traveled farther. Both aircraft fired missiles. The MiG dodged but the slow-moving Predator went down in flames.

And in an ironic preview of things to come, in September 2009 a USAF Reaper drone— a larger version of the Predator, a so-called "hunter-killer" drone—malfunctioned over Afghanistan and flew toward Tajikistan. Rather than risk the unmanned aerial vehicle falling into foreign hands, the Air Force sortied an F-15E to shoot down the drone.

With more and more countries adopting bigger and better drones—and arming them— mid-air clashes are becoming common. China, Iran, and the United States are all enthusiastic exporters of large, weaponized drones.

At last count, no fewer than a dozen countries possess Predator-style drones armed with bombs or missiles. Many non-state groups, including Hezbollah and Islamic State, have bought similar UAVs or improvised their own by strapping small explosives to commercial-style quadcopters.

Read more: The Islamic State Is Pioneering a New Type of Drone Warfare

But the current generation of killer drones is propeller-driven, slow, lacking in maneuverability and still relatively unsophisticated compared to traditional fighters.

"Current military UAVs, including those in the US arsenal and the Iranian Shahed that was recently shot down, look like sitting ducks to a manned fighter aircraft," Todd Humphreys, a professor and drone expert at the University of Texas at Austin, told Motherboard.

The Pentagon got so worried about its Predators flying near the Iranian border that, in 2013, it began sending F-22 stealth fighters to escort the Predators and intercept any Iranian jets that tried to shoot down the slow-flying drones.

The aerial balance of power could tip in coming years. The US, Russia, China, Iran, and several European countries are all developing armed, jet-powered drones that are faster, more maneuverable, and more heavily-armed than current models.

"UAVs will eventually achieve superiority," Humphreys said. "They'll pull Gs far beyond what a human can withstand, and they'll react so quickly and coordinate among themselves so effectively that it'll be the manned fighter aircraft that become the sitting ducks."

"I suppose the transition to UAVs being superior in dogfights will happen sometime in the next 20 years," Humphreys added.