WASHINGTON — U.S. special operations forces are on the ground alongside Syrian Kurdish allies in Raqqa where they've begun their advance into the Islamic State's de-facto capital, an important development highlighting how American military muscle will shape what's expected to become a grueling urban campaign.

Some 2,500 ISIS fighters remain inside the city, U.S. officials estimate. On Thursday, Kurdish militia — backed by coalition air power and American combat advisers — seized much of eastern Raqqa's al Mishlab neighborhood and established their first foothold there, according to the Syrian Observatory for Human Rights. The U.S. military later confirmed its movement.

"Coalition SOF are in Raqqa, and they are close to the front lines," said Col. Ryan Dillon, a spokesperson for the U.S.-led coalition battling ISIS in Syria and Iraq. The Americans are not "kicking down doors," Dillon added. Rather, their primary mission is to advise partner forces, though they are authorized to defend themselves.

Video: Why U.S. troops in Mosul are wearing Iraqi uniforms

Video: Why U.S. troops in Mosul are wearing Iraqi uniforms

The revelation fits a growing pattern in the ISIS war. As operations intensify in and around key objectives and densely populated urban centers, U.S. commanders send advisers considerably closer to the action to bolster partner forces doing much of the fighting.

For instance, as the battle to liberate Mosul in Iraq moved deeper into the city, Navy SEALs began wearing black fatigues to blend in with Iraqi counter-terror operatives going street by street, and house by house, to flush ISIS fighters into the open.

Heretofore in Syria, U.S. advisers have remained comparatively far from the front lines. During the Kurdish-led offensive to liberate Manbij, American troops mostly stayed at a headquarters outside of the city limits, according to a western volunteer who fought in the battle alongside the People’s Protection Unit, or YPG.

"They stayed with the command element, they weren’t actually fighting," this individual told Military Times on the condition of anonymity. U.S. advisers would provide medical assistance and helped coordinate airstrikes and mortar fire, the source explained, but "they weren’t putting rounds down range."

In Raqqa, special operators embedded with the Kurds are "supposed to go to the last cover-and-concealed position from the front line of advance of the partner force," Dillon said. Even still, such proximity to intense urban fighting means they will likely be engaged in combat.

For U.S. military advisers in conflict zones around the world, the "front lines" increasingly are blurred.

In Somalia, for example, where American troops are supporting local partner forces battling the al-Shabaab terror group, Navy SEAL Kyle Milliken was killed in early May while on an advise-and-assist mission. According to The New York Times, he was not behind the last covered-and-concealed position while Somali forces carried out the raid — contradicting the Pentagon's claim.

As U.S. allies on the ground have closed in on the Islamic State's strongholds, the fighting has become more costly. Iraqi special forces have suffered a 40 percent casualty rate liberating Mosul, according to a U.S. report. Kurdish allies assaulting Raqqa face similar risk, officials have said.

"We expect this to be a fight very similar," Dillon said. "Not quite as built-up and as a dense urban terrain as we've seen in Mosul, but nonetheless ISIS has had almost three years to prepare for this fight, and we expect it to be very difficult."

Raqqa is surrounded by the Syrian Democratic Forces. The only escape route requires a boat to cross the Euphrates River. Over the last several days, coalition aircraft have destroyed nearly 20 boats and barges, killing ISIS fighters attempting to flee the city, Dillon said.

Shawn Snow is a staff writer and editor of The Early Bird Brief. On Twitter:  @SnowSox184