The Air Force pilot carefully throttled the controls of a missile-firing MQ-1 Predator drone flying half a world away in northern Syria. Suddenly his headset crackled to life.
Militants firing from bombed-out buildings had ambushed a U.S.-backed militia on a rubble-strewn street in Raqqah,
The pilot studied the surveillance video streaming onto his screen. A captain, he instructed the staff sergeant at his side to set the drone's target sights and powered up a Hellfire missile under its wing.
"Rifle," the pilot said, and the missile soared away.
"Splash," he said seconds later as a fireball swelled across the screen.
The July 18 airstrike was delivered within 160 feet of the pinned-down troops from the Syrian Democratic Forces, according to Air Force officials. It thus marks an evolution in warfare.
American drone pilots now routinely launch missiles at what the Pentagon calls "danger-close" distances to U.S.-backed rebel ground forces fighting Islamic State in densely populated cities.
Hundreds of U.S special operations forces are deployed in Syria, and in some cases they direct airstrikes. But the danger-close missions also require approval from Syrian militia commanders because the missile blasts may put their ground troops at risk.
"Ideally you don't want to accept that level of risk unless you have to," said Col. Julian C. Cheater, commander at Creech Air Force Base, where most U.S. Predator and Reaper drone pilots are based. "But in an urban fight — like you're now seeing in Raqqah — options might not be available to you."
Over the last 20 years, unmanned aircraft were primarily used to collect intelligence or to launch Hellfire missiles at specific terrorist targets after extensive surveillance — enemy strongholds or targeted killings of suspects in Iraq, Afghanistan, Pakistan, Somalia and elsewhere.
But drones are no longer confined to shadowy counter-terrorism missions. Their role has expanded to include more traditional military operations, including an increase in airstrikes during combat and close support for advancing ground troops.
In Raqqah, drone strikes have helped dislodge and kill militants in the city's narrow streets and crowded warrens, where they hid snipers, booby-traps and car bombs.
"Day by day, we are controlling more area," Haval Bilind, a Syrian Democratic Forces commander, said by phone. "Recently enemies attacked our forces from different sectors. The coalition dealt with it very well. The airstrikes were close to our forces, but no casualties."
About a dozen Predator and Reaper drones hunt for targets in Raqqah each day. The pilots and sensor operators work from dimly lit trailers here at Creech, a sun-scorched base 45 miles northwest of Las Vegas.
The crews launch airstrikes from drones or pass the targets to coalition manned warplanes that constantly circle above the city, like vultures wheeling over a carcass, in a tiered system that U.S. air commanders call a stack.
The air campaign has helped the Kurdish-dominated Syrian Democratic Forces capture more than half of Raqqah since a three-pronged ground assault was launched two months ago, U.S. and Syrian officials say.
To assist the drone pilots, some Syrian Democratic Force commanders have been given a device called the ROVER, for remote operated video enhanced receiver. It displays real-time feeds from the cameras and sensors flying above them.
"They have access to our video and are able to tell us exactly what to look at," said one of the drone pilots, who spoke on condition he not be identified. "Because of that, it gives them greater peace of mind and greater confidence as they carry out their missions."
The Air Force launched its first danger-close drone strikes while aiding local forces fighting Islamic State in Surt, a coastal city in Libya, last year.
Commanders had little choice — they didn't have enough manned warplanes in the area.
Drones subsequently launched more than 70% of 495 airstrikes against the militants, helping local Libyan militia fighters retake Surt by December, according to the Air Force.
Danger-close drone missions also played a role in the nine-month offensive to retake the Iraqi city of Mosul, which finally fell to coalition-backed Iraqi forces July 10.
Commanders replicated the tactic with the assault on Raqqah.
Islamic State fighters spent three years fortifying the city after they captured it in January 2014 and turned it into the capital of their self-declared Islamist caliphate. They built a network of defensive positions, dug extensive tunnel systems and rigged buildings with mines and other booby traps.
The result has been a grinding battle of attrition, with house-to-house fighting and U.S.-backed forces trying to block militants from escaping. Brett McGurk, U.S. special envoy for the coalition, said this month that about 2,000 Islamic State fighters remain in the city and "most likely will die in Raqqah."
Coalition aircraft have dropped 3,200 guided bombs and missiles on Raqqah so far, according to the Air Force. Human rights groups say the air war has jeopardized civilians trapped in the city, and United Nations investigators say hundreds have died.
Airwars, a London-based nonprofit group that tracks the air attacks, estimates that the air campaign has killed about 1,600 civilians so far.
"We have seen a consistent rise in civilian casualties in Raqqah to a degree we have never seen from the coalition before in Syria," said Kinda Haddad, a researcher with Airwars.
Haddad said 30 to 50 airstrikes a week produce civilian casualties. "Some of these are incidents where one or two people are killed, and some are where entire families are wiped out," she added.
Drone pilots at Creech insist they are usually able to avoid civilian casualties because of the flood of data at their fingertips. Their drones can stay aloft 20 hours monitoring a target or a battle zone, and the video and other information appear on multiple computer screens.
Pilots concede that confirmation bias — seeing suspicious activity when it isn't there — can color their judgments and lead to deadly mistakes.
They say they try to guard against errors by exchanging text messages with spotters on the ground or at a command center. They use encrypted phone lines to talk to intelligence analysts and radios to contact ground forces.
Most important, the pilots say, targeting and guidance systems have steadily improved, adding a greater degree of precision than in the past.
"If you have three-story buildings on either side, I can shoot down an alleyway," said a Reaper pilot and operations commander who wasn't authorized to give his name. "If there's a sniper in one of the windows, I can hit that."
After the danger-close missions, he added, "we have a lesson learned on how to pair with those ground forces and take the city and those civilian centers back."
Special correspondent Wael Resol in Irbil, Iraq, contributed to this report.