BAGHOUZ, Syria — U.S.-backed fighters could soon resume an offensive against the Islamic State in the last area they control in Syria, an official said Saturday, adding that the battle against the extremists is expected to take three days.

Aras Orkesh of the Kurdish-led Syrian Democratic Forces said they have about 2,500 fighters ready for the battle in and around the eastern village of Baghouz, where Islamic State fighters still have a presence.

Under cover of heavy coalition bombing on March 1-2, SDF forces advanced on the besieged tent encampment, leaving a corridor for residents to leave. After that operation, thousands of residents and many fighters evacuated Baghouz over the next four days. But since Friday only a small group 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 Islamic State fighters.

Orkesh, the SDF official, said the battle could restart “tonight or tomorrow. It’s the end of the cease-fire with Daesh.” He used an Arabic acronym to refer to the Islamic State, 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 said Friday that nearly all of the 20,000, including women and children, are seen as Islamic State followers or adherents.

The official said it would not be a surprise, based on current conditions, if it took another couple of weeks to finish mopping up the Islamic State enclave.

The area on the east bank of the Euphrates River in the province of Deir el-Zour that borders Iraq has been under attack by SDF since September. After capturing all of the surrounding towns and villages over the past six months, the SDF in recent weeks advanced on Baghouz from three sides, besieging it.

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