"They are much more exposed to enemy contact than those in Iraq," US Army Col. Ryan Dillon told reporters at the Pentagon, referring to US and coalition troops that are accompanying local allies in the fight for Raqqa, ISIS' de-facto capital.
Coalition artillery and aircraft have been supporting the Syrian Democratic Forces as they have fought to capture the Syrian city, a battle coalition officials have said will be hard-fought.
Dillon said the US and coalition troops that have entered Raqqa alongside their local allies "are assisting by calling in strikes against enemy forces within the city."
He explained the primary reason for the increased exposure was that US troops are "putting themselves where they need to be" and advising their local allies in smaller formations, closer to front line combat.
They are "much further down into the formations than in Iraq," he said, while noting that this had been the case since at least when the Raqqa offensive began.
Dillon said that the military estimates that there are about 2,000 ISIS fighters remaining in Raqqa, down from about 2,500 when the assault on the city began nearly 40 days ago.
But while he said the Arab and Kurdish fighters comprising the US-backed Syrian Democratic Forces were "moving very, very fast" at the beginning stages of the campaign, Dillon said they had encountered "stiffening resistance" in recent days.
"The SDF have incurred casualties as they have met some stiffening resistance from ISIS as they move closer into the city center," Dillon said.
While SDF troops have encircled Raqqa and breached the wall that surrounds the old city, Dillon said ISIS was using IED belts and increasingly armed drones to maintain their grip on the city.
Lt. Gen. Stephen Townsend, the commander of the international military coalition fighting ISIS, told reporters Tuesday: "ISIS has had more than three years to prepare the defense of Raqqa. While SDF operations are off to a good start, resistance has been stiffening, and we know this is not going to be an easy fight."