BAGHOUZ, Syria－The US-backed Syrian Democratic Forces, or SDF, started an attack to capture the Islamic State group's last remaining shred of territory in eastern Syria on Sunday and were exchanging fire with the extremists, an SDF official said.

"The military operations have started. Our forces are now clashing with the terrorists and the attack started," said Mustafa Bali, head of the SDF media office.

Aras Orkesh of the SDF said a day earlier that the forces have about 2,500 fighters ready for the battle in and around the eastern village of Baghouz, where IS fighters still have a presence.

Under the cover of heavy coalition bombing on March 1 and 2, SDF forces advanced on the besieged tent encampment, leaving a corridor for noncombatants to escape. Following that operation, thousands of residents and many fighters evacuated Baghouz over the next four days. But since Friday, only a small number came out, raising speculations that a renewed military offensive was being planned.

On Saturday, a black flag inside Baghouz could be seen from a distance as well as moving trucks. On the front line, SDF fighters have assumed a relaxed stance with some walking around on rooftops exposed to IS extremists.

Orkesh, the SDF official, said the battle could restart "tonight or tomorrow. It's the end of the ceasefire with Daesh." He used an Arabic acronym to refer to the IS, adding that the battle could take three days.

"What's holding us up is the civilians, we can see them, women and children inside," he said.

In Washington, a senior defense official estimated that nearly 20,000 people, including 3,500 to 4,000 adult males, have emerged from Baghouz since Feb 20. The official, who could not be identified by name under Pentagon ground rules, said on Friday that nearly all of the 20,000, including women and children, are regarded as IS followers or adherents.

Hundreds of IS militants, along with thousands of civilians, mostly family members, have evacuated the IS-held area after the SDF alternately applied military pressure and allowed time for evacuees to come out.

But some IS militants are still clinging to a patch of land inside the village and are vowing to fight.

AP-Reuters