The Pentagon on Tuesday said “hundreds” of U.S. troops were involved in the military campaign to recapture the city of Raqqa from the Islamic State in Iraq and Syria (ISIS).

Department of Defense spokesman Capt. Jeff Davis said that the United States will use AH-64 Apache helicopters and Marine Corps artillery to support Syrian Democratic Forces (SDF) — a group of Kurdish and Arab fighters — as they attempt to retake Raqqa, the de facto capital of ISIS.

“We have there assisting in this battle Marine M777 Howitzers, which are providing a significant enabling capability to our Special Operations Forces and partners on the ground,” Davis told reporters at the Pentagon.

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“The Marines are providing the artillery support, supplement close air support ... and they provide a mobile line called all weather fire support capability.”

The SDF launched the Raqqa attack on Monday at dawn, the start of what Davis said “will be a very, very difficult battle going forward.”

The city in northeastern Syria has been an ISIS stronghold since it was captured in January 2014. The SDF began its efforts to take Raqqa in November 2016 by seizing control of areas surrounding the city.

Davis said the U.S.-led coalition has been encouraging Raqqa’s residents to try to leave the city if they can, and has dropped leaflets advising such a move.

“That has been very difficult, because ISIS is holding people in. We’ve seen reports of public executions of people who are trying to leave,” Davis said.

“It will obviously be an operation that is going to be difficult going forward and there will be a lot of painstaking efforts to try to avoid civilian casualties as they more in.”

Davis also noted that there are still hundreds of ISIS fighters in the city.

Lt. Gen. Steve Townsend, Commanding General of the Combined Joint Task Force Operation Inherent Resolve, said in a statement that the fight for Raqqa “will be long and difficult but would deliver a decisive blow to the idea of ISIS as a physical caliphate.”

“We all saw the heinous attack in Manchester, England,” Townsend said, referring to the terror attack late last month at an Ariana Grande concert, which ISIS has claimed credit for.

“ISIS threatens all of our nations, not just Iraq and Syria, but in our own homelands as well. This cannot stand.”