A fighter from the Syrian Democratic Forces (SDF) gives bread to children near the village of Baghouz, Deir Al Zor province, Syria February 20, 2019. REUTERS/Rodi Said/File Photo

Beirut- Asharq Al-Awsat

A land mine left by the ISIS terrorist group struck a van packed with workers in eastern Syria on Sunday, killing more than 20 of them, Syria's state news agency said. The agency earlier reported that 24 people were killed.

SANA said the explosion on Sunday morning near the central town of Salamiyeh was caused by explosives left behind by the militants when they controlled the area. A mine exploded in a nearby area earlier this month, killing seven people.

SANA said the workers hit by Sunday's blast were heading out to pick desert truffles.

ISIS has been driven from virtually all the territory it once held in Syria and neighboring Iraq, but the extremists left behind countless bombs and booby traps, and large areas have yet to be cleared.

ISIS militants are now cornered by US-backed Syrian forces in a small area near the Iraqi border.

Remnant ISIS militants are besieged in the village of Baghouz, hemmed in by the Euphrates River and the US-backed Syrian Democratic Forces, a Kurdish-led militia spearheading the fight against ISIS following an intense push since September. Thousands of civilians have fled the area held by the extremists in recent weeks.

The presence of so many civilians - and possibly senior members of the militant group - in Baghouz has surprised the SDF and slowed down the expected announcement of the extremist group's territorial defeat.

More than a thousand foreign militants could still be sheltering among civilians in ISIS' final stronghold in east Syria, an official with the US-backed force trying to defeat them said on Sunday.

Throughout its steady advance across the Syrian stretch of ISIS' self-declared caliphate, the SDF has been slowed by the group's extensive use of tunnels and human shields - tactics it says are still being deployed in Baghouz.

"It is expected that there are still undiscovered tunnels, even rooms underground," said Mustafa Bali, an SDF spokesman. "This creates a military problem for us."

The capture of Baghouz will end a campaign of conventional warfare which began in the ruins of Kobani on Syria's border with Turkey in late 2014, when the SDF's strongest component, the Kurdish YPG militia, halted the militant advance.

It was the high water mark for the caliphate that Abu Bakr al-Baghdadi had declared earlier that year from a medieval mosque in Mosul, nothern Iraq, followed by years of steady militant defeats that now culminate in the steppe of eastern Syria.

Despite the loss of its territory in Iraq and Syria, however, local and Western officials warn the group will still pose a threat there, going underground and using guerrilla tactics.

On the long road through SDF territory to Baghouz, there are numerous checkpoints run by the Kurdish Asayish security force, and SDF officials warned of ambushes and bomb-rigged motorcycles.

On the walls of a military base not far from the front line are SDF murals, as well as some vaunting the military feats of the YPG and its all-women counterpart, the YPJ.

Bali said around 6,000 civilians have come out of Baghouz in recent days, many of them militants' wives and children. Some 20,000 had already left over the preceding weeks before the final phase of the siege began.

Some of the fighters have attempted to slip out with them, and the SDF has set up screening points to vet everybody leaving the enclave.

However, Bali said it was hard to predict how many non-combatants remain inside the pocket and that although sources inside Baghouz have said there may be 5,000 civilians, previous estimates have turned out to be wrong.

As for the number of ISIS fighters, "there could be more than 1,000. They are all foreigners", he said.

"They are very fierce and professional, with high levels of experience. These are the elite fighters of Daesh who have gathered here from all over the world," he said, using the Arabic acronym for ISIS.

The operation to pull out remaining civilians in trucks and buses over poor roads was delayed on Sunday because of bad weather, but Bali said it would continue.

"We will not end our moral victory over Daesh with a massacre," he said. "Whatever the price and whatever we can do we will work to evacuate the civilians. After that, the attack. There are two options: surrender or war."