Tens of thousands of dishevelled women, children and men have streamed out of a small pocket in the village of Baghouz near the Iraqi border in recent weeks -- and they still keep coming.
The huge numbers have flummoxed the Syrian Democratic Forces (SDF) and slowed down their offensive aimed at dealing a knock-out blow to the jihadists' once-sprawling proto-state.
"Fighting continues as the #SDF advancing into Daesh encampment," SDF spokesman Mustafa Bali said in a tweet late Sunday.
"Several positions captured and an ammunition storage has been blown up by our forces. SDF is now holding positions inside the camp in #Baghouz."
At an SDF outpost inside Baghouz on Sunday, an AFP correspondent saw white smoke rising above the IS enclave as the sound of airstrikes and shelling rang out.
A large blaze ravaged the makeshift encampment where holdout IS fighters are making a last stand.
The Kurdish-led force, backed by US-led coalition warplanes, has rained fire on the jihadists for a week, blitzing thousands of fighters and family members into surrender.
But SDF spokesman Kino Gabriel told a news conference Sunday that according to the latest group that quit the pocket, "an estimated 5,000 people" are still holed up inside.
He cautioned, however, that the SDF has not been able to verify that figure.
Those fleeing the pocket have previously reported widely inconsistent figures on the number of people still inside, ranging from thousands to a few hundred.
- Surrenders and evacuations -
The SDF spokesman said there was no clear timeline for the end of the operation, estimating that it may take several more days at least before IS is driven from its last bastion.
"I hope it won't take more than a week but this is a personal estimate," he said in the village of Sousa in eastern Syria.
Gabriel said that nearly 30,000 IS members and their relatives have surrendered to US-backed forces since January 9, including more than 5,000 fighters.
An additional 34,000 civilians have been evacuated from the IS redoubt over the same period, he said.
The exodus has sparked a humanitarian crisis in Kurdish-run camps for the displaced further north, where civilians have been transported.
More than 1,300 jihadists have been killed and around 520 have been captured in special operations by the SDF since the battle for Baghouz started, Gabriel said.
The SDF said on Saturday that dozens of jihadists and their relatives had handed themselves over.
They include Europeans, Turks, Chinese, Iraqis and Syrians, it said in a statement.
Die-hard jihadists who stayed behind to defend their last bastion have launched a series of suicide bombings in recent days to hamper the SDF advance.
On Friday, IS launched three suicide attacks outside Baghouz, killing six people as they fled the village.
- 'There is still resistance' -
Ali Khalaf Ibrahim, an SDF fighter, told AFP on Sunday that the jihadists were putting up a strong defence.
"Several times they tried to infiltrate (SDF positions) with explosive belts but they were intercepted" by SDF fighters, he said.
At the height of its brutal rule, IS controlled territory in Syria and Iraq the size of the United Kingdom, with a population of millions.
The total capture of Baghouz by the SDF would mark the end of the cross-border "caliphate" it proclaimed more than four years ago.
But IS still retains a presence in eastern Syria's vast Badia desert and has sleeper cells in the northeast.
Baghouz is the latest front in the Syrian war that has killed 370,000 people since it started in 2011.
Elsewhere in Syria, landmines left behind by IS have killed at least 17 people in a 24-hour period at the weekend, a war monitor said on Sunday.
An ordnance left behind by jihadists exploded in the eastern province of Deir Ezzor on Saturday, killing 16 people and wounding 32 others, said the Syrian Observatory for Human Rights.
A separate landmine explosion in the northern province of Aleppo on Sunday killed one person, the Britain-based Observatory said.
Another two SDF fighters were killed Sunday in a mine blast at Hajin, a town near Baghouz, that was claimed by IS in a statement on Telegram.
At least 44 people have been killed in landmine explosions across Syria in the past three weeks, it added.
In Damascus, the UN envoy for Syria held talks on Sunday with Foreign Minister Walid Muallem, who stressed the need for a Syrian-led political solution to the eight-year war.
Norwegian diplomat Geir Pedersen discussed efforts to find a political settlement to the conflict, including moves to form a committee tasked with drawing up a post-war constitution, state media said.
Pedersen arrived in Damascus on Sunday in the second such visit since he took up his post in January.