Drone warfare recently entered a new chapter in which America's adversaries can successfully deliver explosives via cheap, commercially available unmanned aircraft, challenging the U.S. military to go on the defensive.

An explosives-laden Islamic State drone killed two Kurdish peshmerga troops and wounded two French paratroopers early this month in the vicinity of Erbil, Iraq, Popular Science reported. It is believed to be the first time the militant group has inflicted such casualties.

The drone exploded while it was being inspected after being shot down, Air Force Col. John Dorrian, Operation Inherent Resolve spokesman, confirmed Wednesday.

"We've seen several reports about ISIS use of commercial off-the-shelf drones, including instances where they've used these capabilities to deliver explosives," Dorrian told reporters in a Pentagon teleconference from Baghdad, Iraq.

"It's a threat that's not new to the area," he said, calling the attack a Trojan Horse-style attack.

Nor is it the first time ISIS has tried.

There are at least two other instances over the last month that ISIS has tried to use drones to deliver explosives, according to The New York Times.

The two peshmerga troops were killed Oct. 2, according to a U.S. official, who said the drone looked like a Styrofoam model plane that was taped together in a very rudimentary style, according to The Associated Press. The official said it appeared to be carrying a C4 charge and batteries, and may have had a timer on it, AP reported.

ISIS isn't the only militant group to use drone warfare.

A recently released video belonging to an al-Qaida offshoot, Jund al-Aqsa, purportedly shows a drone landing on Syrian military barracks, AP reported. In another video, small explosives purportedly dropped by the Iran-backed Shiite militant group Hezbollah target the Sunni militant group Jabhat Fatah al-Sham, formerly known as the Nusra Front, near Aleppo. The technology is not new, but the videos are the first known demonstration of these capabilities by any militant groups, according to AP. 

Dorrian noted that the U.S.-led coalition has been actively working to counter enemy drone activity in Iraq.

In addition to unspecified "advanced systems" to defeat drones, he cited the deployment of the DroneDefender, a system which neutralizes in-flight drones with jamming radio pulses.

"We don’t just let the enemy develop a capability that threatens our forces and those forces of our allies and partners and leave that threat unaddressed," Dorrian said.

How to respond to this "red drone threat" has already been on the table for several years, said Rebecca Grant, president of IRIS Independent Research in Washington, D.C.

"In briefings, war games and exercises, a lot of the thinking about how to counter this in the future is not unlike doing counter-battery work, so utilizing the correct radar surveillance and eventually perhaps lasers," she said.

Shooting lasers to defeat drones is not science fiction, however.

The U.S. Navy successfully conducted live-fire tests of a drone-defense laser on the amphibious transport dock ship Ponce in 2014. The Office of Naval Research announced last year that it would begin mounting lasers, or "Ground-Based Air Defense Directed Energy On-the-Move," on Humvees to defeat drones on the battlefield.

Most recently, the Defense Advanced Research Projects Agency asked for help in August in developing on new ways to defend against small drones within the next four years.