The Mosul battle in Iraq has seen the Islamic State group increasingly resort to weaponised drones, which Western governments fear could lead to a new type of attack at home. France issued an internal note to its security forces last week warning that "this threat is to be taken into account nationwide" and ordering any drone be treated as a "suspicious package".
The first record of a deadly IS drone attack was in October when two Iraqi Kurdish fighters were killed and two French special forces soldiers wounded. The device had been booby-trapped and did its damage on the ground when forces approached it after it landed.
"The use of drones by terrorist and insurgent forces is a growing issue of international concern," James Bevan, executive director of the Conflict Armament Research NGO, wrote in a recent report.
Western countries have seen an unprecedented wave of attacks perpetrated or inspired by IS and the new airborne threat is giving chills to security agencies.
WATCH: Iraqi forces discover #ISIS drone factory in #Mosul https://t.co/pfBYxl2bX2#TwitterKurds pic.twitter.com/z7x0hQZv1C — Kurdistan24 English (@K24English) November 26, 2016
Forces battling their way to the outskirts of Mosul have reported several similar incidents. "They are also using drones in this area," Abu Mohammed al-Atabi, a commander with the Hashed al-Shaabi paramilitaries deployed southwest of Mosul told AFP last week.
A high-ranking army officer posted on the southern front said his soldiers were attacked by a modified Phantom 4, a basic camera-fitted "quadcopter" that can be purchased online for less than $1,000.
Experts argue that, compared to the suicide car and truck bombs IS sometimes fills with several tonnes of explosives, drones represent a minor threat.
IRAQ: ISIS claim to have shot down a US MQ-9 Reaper drone near Tal Afar, west of Mosul. pic.twitter.com/JgdwWfxjnM — Conflict News (@Conflicts) November 21, 2016
He warned that could soon change, however. "Future off-the-shelf drones will be able to carry heavier payloads, fly and loiter longer, venture farther from their controller and be able to do so via more secure communications links," he said.
The disaster scenario is one in which IS uses drones to disseminate the kind of chemicals it has so far used with limited success on rockets.
"Although technically much more difficult to achieve, aerosol or spraying devices can also be attached to a UAS to distribute chemical and biological agents," Rassler said.
To counter this new threat, some Western countries have started developing defence systems capable of spotting, tracking and destroying drones.
The US military is using kinetic anti-drone systems that physically take on the devices, while others favour hacking or scrambling.
Another more unusual technique developed in France uses eagles that are trained -- by being fed meat on drones -- to spot the aircraft and take them down.
"They are capable of detecting them from thousands of metres (yards) away and neutralising them," French air force general Jean-Christophe Zimmermann said.