Warplanes and drones circled over the besieged village of Baghouz.
The numbers have flummoxed the Kurdish-led forces and bogged down their offensive to finish off the once sprawling proto-state.
But after that human flow slowed to a trickle, the Kurdish-led force late yesterday told remaining IS fighters time was up for any surrenders and they were moving in.
Earlier on Sunday, a Reuters correspondent saw SDF forces advance into a tented area of Baghouz after Daesh fighters withdrew from it. SDF fighters gathered some ammunition and rifles left behind by the extremists.
"We have been consistently wrong, as have our SDF partners, on how big this is in terms of both fighters coming out and also what we consider ISIS affiliates, these are women and children", the official said.
On Tuesday about 500 ISIS fighters surrendered to SDF forces. They packed high-caliber bullets into munitions belts and headed to the front line.
Shiekh said that to minimize casualties the SDF will advance on the ground, with the USA -led coalition providing air support.
It was not clear Sunday how many fighters and civilians remained in the half-mile-square pocket under ISIS control in Baghouz.
The attack is aimed at wiping out the last vestige of the group's self-declared "caliphate" that once spanned one-third of Iraq and Syria.
Backed forces launch assault on IS group's last Syrian enclave
The SDF has been marching toward Baghouz since September.
But their home countries have mostly been reluctant to take them back, with Britain stripping several women who have joined IS of their nationalities.
Syrian official news agency SANA reported on Monday that more than 50 civilians, including women and children, had been killed as a result of a US-led coalition airstrike on Baghouz. But since Friday, only a small group has come out, raising speculation that a renewed military offensive is about to begin.
An SDF commander who identified himself as Bilal said Sunday's fighting "was the heaviest clashes he'd seen since then".
The official who could not be identified by name under Pentagon ground rules, said that almost all of the 20,000, including women and children, are seen as Daesh followers or adherents.
Syria's Kurds have played a major role in fighting Islamic State in the war-torn country.
Syrian opposition activists say dozens of people have demonstrated in the southern city of Daraa to protest the construction of a statue of late Syrian President Hafez Assad.
Pro-Syrian government forces hold the opposite bank of the Euphrates across from Baghouz and Iraqi militias are stationed at the border, cutting off any easy escape route for the jihadists.
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