The wreck of an Islamic State drone captured by Iraqi special forces in Mosul, Iraq.

U.S. airstrikes across eastern Syria have hobbled Islamic State's deadly drone program, U.S. officials say, but commanders warn that proliferation of the inexpensive technology may allow terrorist groups to launch other aerial attacks around the globe.

U.S.-backed fighters have reported small drones flown by the militants seven times this month in Iraq and Syria as Islamic State struggles to maintain the crumbling borders of its self-declared caliphate, according to the U.S. military task force in Baghdad.

That’s down from more than 60 drone sightings earlier this year, especially during the battle for the Iraqi city of Mosul, which was liberated in early July. Dozens of Iraqis were killed or wounded by 40-millimeter grenades and light explosives dropped from remote-controlled devices that one U.S. commander likened to killer bees.

The use of camera-equipped quadcopters and model-plane-sized drones, sometimes flying in swarms, had become a signature tactic of Islamic State, much as the growing U.S. fleet of large missile firing Predator and Reaper drones have changed the face of modern warfare.

Islamic State had a separate division to purchase the drones from commercial websites and other sources in China, India and Turkey, according to U.S. officials. Engineers in Islamic State upgraded the power systems so the devices could fly longer and drop crude munitions on opposing forces.

Over the last two months, U.S. warplanes have destroyed several Islamic State drone depots, machine workshops and pilot schools.

The airstrikes also killed eight commanders said to be responsible for obtaining, arming and distributing the drones down smuggling routes between Iraq and Syria.

U.S. authorities also recovered several downed drones, and obtained GPS data from the targeting systems that showed where they were launched, according to U.S. officials who were not authorized to speak publicly on intelligence matters.

Col. Ryan Dillon, the Baghdad-based spokesman for the campaign against Islamic State, said the U.S. sought to remove the militants’ “tactical ability to get their systems airborne.”

“We achieved the greatest effect against their program by simply killing the people that have weaponized it,” he said.

The growing availability and sophistication of drone technology pose a broader threat to U.S. counter-terrorism agencies as well as the military.

Islamic State affiliates in the Philippines, Libya and Yemen already have used drones for surveillance. So have the Al Qaeda-affiliate in Syria, Hezbollah in Lebanon, and the Taliban in Afghanistan.

Nicholas Rasmussen, the director of the National Counterterrorism Center, told Congress on Wednesday that his agency has been coordinating with law enforcement and aviation regulators to study ways to defend against small drones being used in terrorist attacks.

“Two years ago, this was not a problem,” he told the Senate Committee on Homeland Security and Governmental Affairs. “A year ago this was an emerging problem. Now it’s a real problem.”

Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, told Congress on Tuesday that the Pentagon had made slow progress in countering Islamic State's drones.

“It does create a significant challenge, and we have done all we can do today to deal with that challenge, as well as develop the capabilities we'll need tomorrow," he told the Senate Armed Services Committee.

Drones, he added, are "at the top of our list for current emerging threats."

The threat spurred the Army to issue a handbook in April to urge commanders to "employ dedicated observers" and to train soldiers in what it called "Counter-Unmanned Aircraft System Techniques."

Gen. Raymond Thomas, head of Special Operations Command, said small drones were the "most daunting" threat his commandos faced on the battlefield last year.

He recalled once during the battle for Mosul when the Iraqi forces’ “effort nearly came to a screeching halt” because the sky was filled with buzzing robotic aircraft.

“At one point there were 12 killer bees, if you will, right overhead,” he said during a conference in Tampa, Fla., in May.

Don Rassler, an analyst with the Combating Terrorism Center at West Point, said Islamic State used various drones to overwhelm defenses as a deliberate tactic, part of a multiyear, detail-oriented military strategy.

Rassler and two colleagues studied, and subsequently published, internal Islamic State documents about the drone strategy that were recovered from Mosul.

The militants issued standardized “drone use reports” for pilots to fill out after each mission. The pilots were in a special unit, called the Baraa bin Malek Brigade, that was subordinate to Islamic State’s research and development arm, known as the Committee of Military Manufacturing and Development.

“The Islamic State established a bona fide bureaucracy to use drones on the battlefield,” Rassler said. “They have now shown other groups what is possible in using this technology.”

The Pentagon has rushed electronic jammers and other specialized equipment to help Iraqi security forces shoot down or neutralize the militants’ drones.

The Pentagon has also launched multimillion dollar programs to find new solutions, including lasers that can disable a drone in the air, and guns that fire small nets to nab them mid-flight.

Few of the gee-whiz measures have produced tactical success on the battlefield. Iraqis and U.S.-backed Syrian forces have instead tried to shoot them down with automatic weapons.

Peter W. Singer, a fellow at the nonprofit New America Foundation in Washington and author of “Wired for War,” a book on robotic warfare, said the Pentagon was caught flat-footed and scrambling for solutions. Terrorist groups will take note of that, he said.

“This is a threat that will only increase in the coming years,” he said. “The genie is out of the bottle.”

william.hennigan@latimes.com

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