Over the past week, thousands of women and children have poured out of the village of Baghouz, but an unknown number still remain.
They have been surrounded in a tent city on a plot of farmland on the edge of the village, along with a few hundred mostly foreign jihadis who are refusing to surrender.
The Syrian Democratic Forces (SDF), a US-backed militia that is leading the fight against the extremist group, said it expected to evacuate the last of the non-fighters by the end of Thursday, and that an offensive would follow.
But the evacuations did not go ahead as planned, and trucks that had been brought to bring civilians out left the area empty.
“We can’t get into details, but today no civilians came out,” SDF official Mustafa Bali told Reuters.
Some 2,000 women and children, and dozens of suspected male fighters, left the area on Wednesday, following in the footsteps of some 20,000 who have fled in the past three weeks alone.
Those who have managed to escape the desolate camp in recent weeks have arrived on the other side of the frontline in poor health and suffering from malnutrition.
Up until now, suspected fighters have been taken to prisons run by the SDF, while women and children go to camps for processing and questioning. But on Thursday the SDF took the unusual step of handing over 150 mostly Iraqi fighters to the Iraqi army.
The deal, first reported by Reuters, appears to be an attempt by the SDF to relieve pressure on its overcrowded jails, which are packed with thousands of captured Isis fighters.
The Kurdish-led group has spoken out in recent weeks over the refusal of European countries to take back citizens who came to Syria and Iraq to join Isis, arguing that it does not have the resources to hold the fighters and their families indefinitely.
“Early this morning, 10 trucks loaded with Daesh fighters and their families were handed over by SDF forces to the Iraqi army,” Ahmed al-Mahallawi, the mayor of Iraqi border town Al-Qaim, told Reuters.
“The majority of them are Iraqis and the convoy was under maximum security protection headed to the Jazeera and Badiya military headquarters.”
There are thought to be at least six British fighters currently held by the SDF, and around 20 women and children. The UK has so far refused to take back any of the detainees, including Shamima Begum, a 19-year-old British woman who was one of three schoolgirls from east London who went to join Isis in 2015.
Iraq has been criticised by Human Rights Watch for conducting “show trials” of suspected Isis members. The Associated Press previously reported that Iraq had detained at least 19,000 people for alleged Isis ties, and sentenced more than 3,000 to death.