Islamic States fighters are flying off-the-shelf drones as artillery spotters in the defense of Mosul, the U.S. and coalition commander for operations in Iraq and Syria said Wednesday.
The small drones have been used "to control and direct indirect fire" by artillery and mortars, said Army Lt. Gen. Stephen Townsend, commander of Combined Joint Task Force-Operation Inherent Resolve.
The drones have gone from being a nuisance to a battlefield threat to U.S., coalition and local forces, Townsend said in a video briefing from Baghdad to the Pentagon.
"It's a pretty thorny problem" and "we expect to see more of this" as the advance on Mosul continues against the Islamic State of Iraq and Syria, he said.
ISIS has a history of using drones and model airplanes, mostly to conduct surveillance. But the group recently has been rigging them to drop small explosive charges, Townsend said. It has used drones to drop explosives against the Mosul offensive but no casualties have resulted, he added.
Townsend also referred to the incident earlier this month in which ISIS used a model airplane as what he called a "Trojan horse" against Kurdish Peshmerga fighters in northern Iraq.
The model airplane essentially was an airborne improvised explosive device fitted with an explosive charge and a timer, according to Pentagon officials. Two Kurdish Peshmerga fighters who brought down the drone and were examining it were killed when the charge exploded.
The use of the drones is not "episodic" and poses a continuing threat, which now includes using the technology as artillery spotters, Townsend said.
The U.S. has fielded several systems he declined to describe to counter the drones by jamming, and friendly forces have also been told to bring them down with small-arms fire.
In July, the Pentagon announced that $20 million was being put into counter-drone measures, including systems to block GPS and jam radio signals.