U.S.-backed forces said Tuesday that they have liberated the Islamic State in Iraq and Syria’s (ISIS) self-declared capital of Raqqa in northern Syria.

There are roughly 6,500 ISIS fighters left in both Syria and Iraq, and only about 100 are “all but isolated in their quickly shrinking territory” of Raqqa, said Army Col. Ryan Dillon, a spokesman for the U.S.-led coalition fighting the group.

The Syrian Democratic Forces (SDF) on Tuesday declared that Raqqa had been liberated and fighting has ended. The group, a U.S.-backed mix of Kurdish and Arab fighters, has been slowly taking back the city from ISIS over the past four months.

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Dillon, who spoke to reporters at the Pentagon from Baghdad via video feed, said ISIS was on the “verge of a devastating defeat” and more than 90 percent of the city has been cleared, but U.S.-led forces are continuing to oust the fighters.

“We are aware of the reports that ISIS has been defeated in Raqqa,” Dillon said. “However, clearance operations continue, and we expect our Syrian Democratic Force partners to hit pockets of resistance as the final parts of the city is cleared.”

Senate Armed Services Chairman Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.) congratulated U.S. and coalition forces on the announcement Tuesday, calling it “a major moral victory in the fight against ISIS,” but pointed to challenges ahead.

“Terrorists inspired by ISIS remain an enduring threat throughout the region; the Syrian Civil War continues to rage just beyond the areas retaken by the coalition; Bashar al-Assad remains in power with the backing of Iran and Russia in an alliance that is hostile to peace, stability, and American interests; and the humanitarian devastation and lack of political progress wrought by six years of war leaves fertile ground for future insurgencies,” McCain said in a statement.

McCain said the United States for too long "has approached the Middle East through the narrow vantage point of counterterrorism." The GOP senator called for the U.S. to adopt "a comprehensive strategy that takes all regional factors into account."

Committee member Sen. Ben Sasse Benjamin (Ben) Eric SasseChamber of Commerce endorses McSally for reelection Ben Sasse is mistaken with idea for the election of senators in America Big Ten football to return in October MORE (R-Neb.), meanwhile, said in a statement that the victory "underscores the need for a comprehensive Syria strategy because our military, our partners and allies, and those brave souls who are literally fighting for their homes and for their neighbors deserve nothing less than our full commitment to see this through."

Dillon said the SDF will now look to an area known as the Middle Euphrates River Valley in Syria's Deir ez-Zour province, where they have already been fighting ISIS.

“We have to wait and see as things develop,” Dillon said, adding that there appeared to be a desire by the SDF to push further down the Euphrates.