TAMPA, Fla. — Islamic State group militants used armed, commercially available drones to bedevil U.S. and coalition forces fighting in Mosul in 2016, a vexing problem that the four-star head of U.S. Special Operations Command said was the "most daunting" threat his operators faced on the battlefield last year.   

"This last year's most daunting problem was an adaptive enemy who, for a time, enjoyed tactical superiority in the airspace under our conventional air superiority in the form of commercially available drones and fuel-expedient weapons systems, and our only available response was small arms fire," Gen. Raymond Thomas told a packed Special Operations Forces Industry Conference on Tuesday.

Thomas told the story to highlight how ISIS militants have been able to adapt to U.S. tactics and strengths and find creative ways to harass U.S. and Iraqi forces. The evolving tactics forced U.S. forces to adjust on the fly and to find and rapidly field technologies to combat the threat.

The drone threat became so serious that at one point it nearly stalled the Iraqi offensive in Mosul, Thomas said.

"About five or six months ago, there was a day when the Iraqi effort nearly came to a screeching halt, where literally over 24 hours there were 70 drones in the air," Thomas recounted. "At one point there were 12 'killer bees,' if you will, right overhead and underneath our air superiority."

The drones were quadcopters available off the shelf for $2,000, to which ISIS attached a weapon that fired a 40 mm munition.

U.S. forces got to work attacking the drone operators’ support network and communications, but they also sought out electromagnetic and radio frequency technologies to counter drones, which got to the field quickly, Thomas said.

"It’s not defeated yet," Thomas said. "I would not go to sleep on this enemy ever."

"They are going to adapt and figure another way to come at us. But this was a momentary advantage they had that we had to rectify."

David B. Larter was the naval warfare reporter for Defense News.