A leading defence think tank says the proliferation of armed drones is creating significant security challenges, particularly where states, notably Iran, use the build-up as an instrument of foreign policy. The International Institute for Strategic Studies highlighted the spread of unmanned aerial vehicles (UAVs) in its 2022 <i>Military Balance</i> survey of the global security landscape. The report said Yemen's Iran-backed Houthis “demonstrated increased missile and uninhabited vehicle capabilities” in dozens of attacks last year. John Chipman, the director general of the IISS, said there were growing risks to civilian infrastructure, as well as to military targets in this Iranian policy. He spoke of incidents recently reported in the Emirates. “The UAE attacks carried out by Yemen's Houthi rebels against oil and aviation facilities also highlight how the distribution of these UAV capabilities to non-state actors can be a tool of state policy, in this case for Iran, a key supporter of the Houthis,” he said at a launch event in London. “Iran has previously transferred UAV capabilities to Hezbollah in Lebanon and to the Syrian government and Iran has also transferred ballistic and cruise missiles, including to the Houthis.” Mr Chipman said the issue was a global concern, with armed UAV systems now in the inventories of at least 20 countries, while others are seeking to acquire these systems. “These range from small hobbyist-style systems employed by some non-state groups to large long-range platforms capable of being fitted with sensors and air-to-surface weapons,” he added. Experts at the London institute highlight the issue of scale in the use of drones in particular. “The challenge of defending against UAVs will only intensify as they evolve, the latest test being the emergence of hybrid systems, which combine the characteristics of a UAV and of an air-to-surface weapon,” Mr Chipman said. “Armed UAVs are no longer the exception, they are rapidly becoming the norm. Indeed, a problem for defending forces is the sheer variety of threat systems that can now be employed by some state and non-state actors, including high, low, fast and slow weapons. "If these are co-ordinated, as in recent Houthi attacks, the challenges are sharper still. “Responding effectively places a premium not only on layers of defensive weapons, but also on the cost of sensor systems and computing power needed to effectively address these threats. "Not all states will be able to afford investments either in the weapons or in the systems needed to counter them.” Doug Barrie, the institute's military aerospace expert, said the variety of drones that can be used at the same time meant their users could acquire attacking threat without spending a great deal of resources. “We've seen the emergence of a subclass of UAV,” Mr Barrie said. “If you'd like the Costco-Poundland cruise missile, where a comparatively simple vehicle is fitted with a warhead. “It makes it simpler but it does provide the ability to attack area targets, such as an oil installation or an airfield. The propaganda value of such attacks generally far outweighs any damage actually achieved. “The use of what are sometimes called lethal UAVs, or direct attack munitions, has on occasion been carried out in concert with ballistic and cruise missile attacks. "Such co-ordination complicates the task of defending against an attack, because you’ve got to identify, track and engage targets across a broad range of altitudes and speed regimes. “For air defenders this is already a wicked problem, and it's one that is only likely to become yet more demanding.”