The SDF have slowed down their offensive as they seek to evacuate civilians before a final push to defeat the jihadists, who seized swathes of Syria and Iraq in 2014 and declared a cross-border caliphate.
The militants are holed up in a civilian camp on the outskirts of Baghouz Village.
Isis fighters had snuck into an evacuation convoy of around 15 trucks, mostly carrying women and children, and were later identified and arrested at a screening checkpoint.
Dozens of trucks earlier headed towards Baghouz, a village on the banks of the Euphrates at the Iraqi border where hundreds of Islamic State militants were making their last stand.
"The crossings opened by the regime and the Russian military are claimed to be safe, but the people here are not feeling any sense of safety or stability", one resident said.
"There are still civilians inside", Bali said.
The civilians who were in the enclave were believed to be mainly family members of IS militants, many of them foreigners.
"They've got a lot of civilians in there, they hold some Syrian Democratic Force prisoners and they are using them as human shields", Votel said of Islamic State.
But by nightfall the trucks had not departed, and reporters there witnessed two air strikes against the jihadists.
SDF and coalition officials estimate there are still about 300 IS fighters in Baghuz, and there have been ongoing attacks by small groups of IS fighters - some thought to be part of sleeper cells - on SDF positions throughout the area.
SDF officials did not say how many were in the convoy or how many remain in the pocket.
"SDF made final warning to remaining militants to surrender", he added.
US President Donald Trump on Saturday, February 16, urged European powers to take back hundreds of their citizens who fought for ISIS in Syria.
Any such pullout would leave Syria's Kurds exposed to a long threatened Turkish assault, and has forced them to mend ties with Damascus after years of seeking self-rule in the northeast of the country.
Abdulkarim Omar, co-chair of foreign relations in the region held by the SDF, said the SDF had captured around 800 foreign Islamic State insurgents.
The SDF has complained that Western countries are reluctant to take back such people, who are seen at home as a security threat but might be hard to legally prosecute. "We did not know our enemy very well but today we know them intimately".
However, it will heighten attention on the promised withdrawal of some 2,000 USA troops who have deployed into Syria during the fight against IS, and with it the fate of the Kurdish-led region that the US presence has helped to secure.
Kurdish forces say they expect total defeat of IS in Syria by the end of the week.
On Tuesday, regime rocket fire killed four civilians including a child in that jihadist bastion's town of Khan Sheikhun, the Observatory said.
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