FILE PHOTO:Damaged buildings are pictured during the fighting with Islamic State's fighters in the old city of Raqqa, Syria, August 19, 2017. REUTERS/Zohra Bensemra/File Photo

BEIRUT (Reuters) - U.S.-backed Syrian militias have taken the historic old city of Raqqa and its ancient mosque as they press their offensive to defeat Islamic State, the U.S.-led coalition said on Monday.

The Syrian Democratic Forces, an alliance of Kurdish and Arab militias, said last week it had captured the final districts in Raqqa’s old city from Islamic State.

With the help of U.S.-led jets and special forces, the SDF has been battling to capture Raqqa city, which Islamic State used as a base of operations in Syria. The SDF pushed into the city in June after fighting for months to encircle it.

The walled old city lies in the heart of Raqqa, but Islamic State militants still control districts in the west of the city. The SDF says it holds 65 percent of Raqqa in total.

“The SDF have made consistent incremental gains in the urban terrain of the city, fighting block by block,” said U.S. Army Colonel Ryan Dillon, a spokesman for the United States-led coalition against Islamic State.

The statement said the SDF has cleared the Great Mosque of Raqqa, its oldest mosque, describing the advance as “a milestone” in the Raqqa battle.

SDF fighters have waged fierce battles with slow progress against Islamic State in Raqqa’s old city since early July, when the U.S. coalition breached its walls with air strikes.

A war monitoring group, the Britain-based Syrian Observatory for Human Rights, said last week that the SDF held more than 90 percent of Raqqa’s old city.