INCIRLIK AIR BASE, Turkey — Here at this once-obscure air base in southeastern Turkey, the number of U.S. troops has nearly doubled since this the summer, while the fleet of U.S. aircraft deployed here has grown four fold.

And credit for that expansion goes directly to the Islamic State group.

The expanding fight against the militants has transformed this strategic outpost about 100 miles from the Syrian border into a round-the-clock hub for operations in Iraq and Syria.

"This is a very important location on the tip of the spear," Defense Secretary Ash Carter told troops here Tuesday on a visit to Incirlik, the first stop on a weeklong tour across the region by President Obama's Pentagon chief.

At a town hall-style meeting, Carter told troops that the purpose of his trip here and across the U.S. Central Command region later this week is to identify options for expanding the fight against the Islamic State, as the group also is known, even if that means putting more American boots on the ground.

"We're looking at way to do more — in the air, on the ground, in Syria, in Iraq — to hasten the defeat of ISIL," Carter said, referring to the Islamic State by one of its alternate acronyms.

The expansion of operations at Incirlik began in September when the Turkish government lifted its longstanding ban on American use of the facility for combat strike operations. For years, the Turks limited use of the airfield to U.S. intelligence, surveillance and reconnaissance missions.

Now, the roar of fighter jets soaring into the sky southward toward the Syrian border is heard throughout the day and night, and the sprawling tarmac is running out of parking space.

"Incirlik is a large base, but it only has so much concrete to park the aircraft," said Col. Sean McCarthy, commander of the 447th Air Expeditionary Group, which deployed here in October.

The base's strategic location near Syria allows pilots to burn less fuel, spend far more time on station and intensify the air campaign targeting Islamic State militants in their strongholds of northern Syria.

The change is illustrated by a 30 percent increase in bombs dropped in November compared to September, according to the latest U.S. Air Force data.

A fleet of KC-135 Stratotankers run about a dozen sorties per day out of Incirlik, representing about one third of all in-air refueling missions supporting the aircraft involved in Operation Inherent Resolve, McCarthy said.

About a dozen A-10 Warthogs also are based here and flying 10 to 12 sorties per day over Iraq and Syria, McCarthy said.

An Air Force major identified only as Charlie said his sorties typically run between five and eight hours long. He'll spend an hour or two flying out to his designated target area and coordinate with a joint terminal attack controller, or JTAC, who is in an operations center on the ground overseeing strikes that patch of territory.

Targets can include platoon-sized Islamic State units fighting American-backed Syrian rebel groups on the ground or buildings known to house Islamic State militants.

Other frequent targets include ISIL roadside checkpoints as well as up-armored vehicles or "bongo trucks" used by the extremist group, said Charlie, whose last name is being withheld here at the request of top Pentagon officials out of security concerns.

Targets are selected very carefully to prevent civilian casualties, he said.

"When we work with the JTAC on the ground … it's always a discussion: 'Here's what I'm seeing, this looks like normal patterns of life. Do I see children? Do I see adults? Is there a guy with sheep?' We are always discussing [the risk of civilian casualties] to make sure we have the right target," the major said.

"In a best case scenario, we have an [unmanned aerial vehicle] and we're able to look at stuff with a little more fidelity."

Another wrinkle in the mission: Flying sorties in northern Syria is uniquely complex because the Americans are sharing the air space with Russian aircraft. The Russians have dozens of advanced fighter jets stationed at an air base near the Syrian port city of Latakia.

The U.S. and Russia have overlapping but ultimately different goals in Syria. The U.S. is targeting Islamic State militants, while the Russians are trying to prop up and support the regime of Syrian president Bashar al Assad.

The Russians occasionally strike Islamic State militants who clash with Syrian government forces in the multipolar civil war. But U.S. military officials say those strikes are rare and Russia often targets other Syrian rebel groups threatening Assad's forces.

The U.S. and Russia do not coordinate air strikes, but their two militaries did sign an agreement in October that establishes basic safety protocols designed to avoid mishaps or misunderstandings.

McCarthy said he has encountered Russian aircraft twice while flying an A-10 over Syria. In each case, the Russian aircraft was several miles away.

He said he has not had personal, direct contact with Russian pilots, but maintained close contact with his own tactical commanders to maintain awareness and avoid any in-air mishaps.

"Nothing I ever experienced was hostile," he said.

However, sharing airspace with the Russians can slow things down, McCarthy said.

"I won't lie to you: Dealing with crowded skies does take time," he said in an interview on Incirlik's flight light, as fighter jets took off from the nearby runway. "You do have to ensure that everybody is aware of everybody else's position."

Adding to the tension is Russia's recent installation of S-400 surface-to-air missile systems at its air base in Syria, putting most U.S. aircraft in Syria and Turkey within its engagement zone. But McCarthy said he's not troubled by the increased risk.

"I don't lose any sleep over what they are doing with their surface-to-air missile systems," he said.

Charlie, the Air Force major, also said he's aware of the Russian missile systems but sees no immediate cause for alarm.

"It's there. We're there. We're just sort of doing our thing and so far it's working out pretty well," he said.

To date, his aircraft's sensors have not detected any aggressive or hostile action from the Russian missile system's radar. "I have not seen any locking and painting," he said.

The total U.S. force deployed at Incirlik has grown from about 1,300 earlier this year to nearly 2,500 now.

The U.S. just opened a new dining facility to handle the troop surge, and a new fitness center is under construction, an official said.

But the base's increasingly prominent role in the fight against the Islamic State group has unmistakably raised new security concerns.

In July, worries about extremists targeting U.S. troops and their families prompted military officials to lock down the base and prohibit the roughly 5,000 American troops, civilians and dependents from venturing out to the "American alley" outside the installation's main gate, where troops for years have patronized kabob shops, carpet salesmen and local markets.

Yet there seems to be little worry about any immediate danger among the base's military population.

In September, the Defense Department offered voluntary evacuations to the roughly 900 military dependents at Incirlik, but only about 80, or less than 10 percent, decided to leave, said Kristine Verbeten, a civilian deputy for the 39th Force Support Squadron here.