BEIRUT (AP) — The U.S.-backed Kurdish-led forces fighting Islamic State militants in Syria said on Saturday that they are waging the "final" battle to uproot the extremists from the northern city of Raqqa, once the de facto capital of the militant group.

This final battle could take hours or days, said Mustafa Bali, the spokesman for the Kurdish-led Syrian Democratic Forces.

Losing Raqqa, in many ways the symbol of IS' caliphate, would be yet another blow to the militant group and its reign of terror in the region as its strongholds crumble one after another in Iraq and other parts of Syria.

IS militants are desperately fighting in a number of neighborhoods in the city, Bali said.

"They are still able to fight but they will no longer determine the fate of the battle," Bali said. "Our forces are waging the final phase of the battle of the presence of Daesh in Raqqa. We have not decided if this battle will last hours, days or weeks." Daesh is the Arabic term for IS.

Over the past days, mediation efforts by local tribesmen and the local civil council have secured the release of a number of civilians from Raqqa. A local council official, Omar Alloush of the Raqqa Civilian Council, told The Associated Press on Friday that 100 IS fighters surrendered to U.S.-backed forces.

U.S. officials have said that only surrender, not a negotiated withdrawal for IS, would be accepted.

Hundreds of terrified civilians have been filmed fleeing toward the Syrian Democratic Forces from the last remaining neighborhoods held by IS in Raqqa ahead of the anticipated final battle for Raqqa between SDF and IS. Earlier this week, US officials said an estimated 4,000 civilians are believed still trapped in the city.

A new video that emerged Friday shows desperate, terrified residents emerging from destroyed districts, some of them collapsing on the ground in exhaustion as they arrive, in haunting scenes reflecting their years-old ordeal.

Militants seized Raqqa in 2014, the first city to fall under the full control of the extremist group. It has since become synonymous with IS' rule of terror in the region, with civilians punished in public killings over the slightest perceived offense — videotaped slayings that have shocked the rest of the world. It was also from Raqqa, which became a destination for foreign fighters from around the world, that many of IS attacks in the West were plotted.

The latest battle for Raqqa began in June, with heavy street-by-street fighting amid intense U.S.-led coalition airstrikes and shelling. The battle has dragged on in the face of stiff resistance from the militants and civilians trapped in the city.

On Saturday, the SDF said they seized the al-Nahda neighborhood in Raqqa's western sector. A day earlier, IS carried out an offensive on SDF forces near the city's main hospital, an IS fortified headquarters.

The Syrian Observatory for Human Rights, a Britain-based monitoring group with activists on the ground, said negotiations over the fate of foreign fighters who remain in Raqqa have delayed the final push to regain control of the city.

SDF officials deny there are talks to evacuate IS fighters.