QAYYARA WEST AIRFIELD, Iraq (Reuters) - As the battle for Mosul moves to the narrow streets and densely packed houses of the Old City, U.S. artillery gunners and helicopter pilots supporting Iraqi forces face an age-old problem – how to avoid killing civilians.

Displaced Iraqis flee their homes as Iraqi forces battle with Islamic State militants, in western Mosul, Iraq March 18, 2017. REUTERS/Suhaib Salem

They place their faith in precision missiles which can hit their target with great accuracy. But human instinct also comes into play against an Islamic State enemy which has used civilians as human shields and hides in houses and mosques.

“Our mission is to find and destroy ISIS. We are not here to kill the wrong people,” said Captain Lucas Gebhart, commander of the 4/6th Cavalry’s Bravo Troop of Apache attack helicopters.

The troop is based at this airfield about 60 kms south of Mosul, as is a rocket battery which fires into west Mosul.

A major site at the height of the U.S. occupation, Islamic State captured Qayyara from Iraqi government forces in 2014 and destroyed it. The Iraqis retook it in July last year, and now the U.S. Army is building it up again as a support base for the Mosul operation.

Gebhart, who wore a U.S. Cavalry hat with a crossed-sabre insignia along with his regular uniform, has been here since December. The troop flies close support for the Iraqi army and escorts medical evacuations. It has had more than 200 engagements with Islamic State fighters in that time, he said.

“We fly every day, weather permitting. We are firing missiles most of the time,” Gebhart told reporters.

The Iraqi army started its offensive on Mosul, Islamic State’s last stronghold in Iraq, in October and retook the east side of the city, bisected by the Tigris river, in January. The west, including the Old City, is much harder going.

“The west side is very congested and it will present new challenges for us. We realize the need to be careful as we go forward,” Gebhart said.

One of those challenges is avoiding civilian casualties in a conflict where fighters are mixed in among the population and sometimes hiding behind them.

“Everyone that flies with me are fathers and husbands, so we are very deliberate to avoid casualties we don’t want. We use guided missiles. The things we shoot from an Apache, they go where we want them to go,” Gebhart said.

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Targets are identified and approved by the Iraqi army. But circumstances can change in a moment.

“I have personal experience of human shields. I engaged a target and they pulled a family of women and children out of a house. The missile was already in the air but I was able to move it,” he said.

“You’ve got a little bit of time. If something happens post-missile release, we have procedures to move it.”

Gebhart, aged 32, joined the military as a teenager after the 9/11 attacks on the United States. He served in the 82nd Airborne in Iraq in 2003 before going to West Point and becoming a cavalry officer. He also served two tours of duty in Afghanistan.

“I love my job. I don’t lose sleep over it,” he said.

WE LOVE TO FIRE

In another section of the base, the 18th Field Artillery “Odin” battery operates a High-Mobility Artillery Rocket System (HIMARS), fired off the back of trucks.

On Friday afternoon, the battery fired 10 rockets, each worth about $100,000, in the space of about 20 minutes. They headed skywards in a cloud of white smoke and a flash of fire as a Bob Marley song played from a platoon tent. They would reach their target in east Mosul in about a minute.

Lieutenant Mary Floyd explained that the rockets were GPS-guided. All fire missions were approved by senior officers at the Combined Joint Operations Center and the coordinates were sent to the battery through computers.

“The rockets go really high so we have to clear airspace -– civilian and military -– along the flight path. We have had to end missions because they saw aviation,” she said.

Although rockets are often aimed at targets in built-up, populated areas, the battery was confident they would hit what they intended. If the rockets are off target, they do not detonate, she said.

“They have very, very low collateral damage, so we like to use them a lot,” Floyd said, using the military term for civilian casualties. “When the rockets hit they land at near a vertical angle. That really confines the blast to one house.”

The battery has fired hundreds of rockets since deploying to Qayyara, she said.

“The tempo changes. We’ll go a couple of days without orders. Then we might be firing all night.”

The issue of civilian casualties has dogged the U.S. military during its long wars in Iraq and Afghanistan, from shootings at check-points to drone bombings. In the battle for Mosul, Iraqi and U.S.-led coalition war planes have also been pounding parts of the city.

Figures of such casualties are hard to come by. Washington has stressed its forces take every effort to avoid them.

On Tuesday, a prominent Iraqi politician and businessman, Khamis Khanjar, said at least 3,500 civilians have been killed in west Mosul since the offensive closed in on it.

The U.S.-led coalition said in a statement that up to March 4, it had assessed that “more likely than not”, at least 220 civilians had been unintentionally killed by coalition strikes since the start of Operation Inherent Resolve.

While the men and women of Odin battery were fully aware of the risk, they believe in their work.

“We love to fire. It makes me very happy,” Floyd said. “At night it is very beautiful.”