U.S.-backed forces in Syria have captured the Old City of Raqqa, the latest milestone in their ongoing assault against the self-proclaimed capital of the Islamic State's rapidly shrinking territories, according to a U.S. military statement on Monday.

Kurdish and Arab fighters with the Syrian Democratic Forces secured the neighborhood over the weekend after vanquishing a last pocket of resistance in the city's historic Great Mosque, the statement said.

The capture followed a grinding two-month battle for the neighborhood that has proved the toughest challenge yet of a three-month-old offensive for Raqqa, launched in June and still far from over.

Unlike in Mosul, the Old City does not lie at the heart of Raqqa and its seizure does not signify an imminent end to the fighting, said a U.S. military spokesman, Col. Ryan Dillon.

The SDF now controls about 60 percent of Raqqa, said Dillon, who would not put a timeline on how long it would take to claim the rest but predicted that weeks of fighting lie ahead.

Dillon said the capture does, however, represent a "significant step" in the overall battle for the city, which has turned into a block-by-block advance against a largely unseen enemy.

"This is still not over. It's still going to be very difficult, but we're making progress," he said.

The Kurdish and Arab forces have been advancing simultaneously on multiple fronts, backed by U.S.-led airstrikes and accompanied by U.S. Special Operations forces, in an effort to stretch the resources of the defending militants.

They are being met by barrages of improvised explosive devices and suicide bombers, many of them emerging behind SDF lines from an extensive network of tunnels dug by the militants to defend the city.

The United Nations says there are about 20,000 civilians trapped in the city, and as was the case in the battle for Mosul, they are bearing the brunt of the relentless airstrikes.

Amnesty International said last month that hundreds of civilians had died in airstrikes since the offensive began and that it had documented 176 deaths in June and July.

The toughest battle is expected to be for a cluster of government buildings in the city center, known as the security block, where the Islamic State has prepared its heaviest defenses.

The advance in Raqqa coincides with significant gains by Syrian government forces farther south on the outskirts of the city of Deir al-Zour, now the only major city in either Iraq or Syria that is still mostly under Islamic State control. Syrian army units backed by loyalist militias advanced to within six miles of a garrison of Syrian soldiers that has been besieged on the edge of the city for the past three years.

The simultaneous advances by U.S.-backed and Syrian government forces on the two major northern and eastern Syrian cities set up a potential scramble to control the rest of Deir al-Zour province, which is still mostly under Islamic State control.

The commander of the U.S.-led coalition, Lt. Gen. Stephen Townsend, said last week that the United States is preparing a force of Kurdish and Arab fighters to seize towns along the Euphrates River valley in Deir al-Zour province, where the Islamic State has been concentrating most of its manpower and resources.

Syrian President Bashar al-Assad has also reiterated his determination to regain control of all of Syria in recent speeches, and it is unclear which forces will arrive first for the final battles against the Islamic State.

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