Dozens of bus passengers were killed in ambush in eastern Syria, state media and a war monitor said, with conflicting accounts of whether they were civilians or government fighters.
The Syrian Observatory for Human Rights said 37 soldiers died in the attack carried out by ISIS in Deir Ezzor province. It said the soldiers were ambushed as they were travelling home for holidays.
Syria's official Sana news agency reported that a "terrorist attack" on a bus killed "25 citizens" and wounded 13.
Two other buses that were part of the convoy managed to escape, according to the Observatory, which relies on sources on the ground in Syria.
ISIS overran large parts of Syria and Iraq and proclaimed a cross-border "caliphate" in 2014, before multiple offensives in the two countries led to its territorial defeat.
The group was overcome in Syria in March last year, but sleeper cells continue to launch attacks, mainly in the vast desert that stretches from the central province of Homs to Deir Ezzor and the border with Iraq.
Rami Abdel Rahman, head of the Britain-based Observatory, said the bus was attacked near the village of Shula by extremist fighters. They detonated roadside bombs before opening fire on the soldiers, who belonged to the army's elite 4th division.
There was no immediate claim of responsibility for the attack.
According to Deir Ezzor residents and intelligence sources quoted by Reuters, there has been a rise in recent months of ambushes and hit-and-run attacks by ISIS militants who hide in caves in the mainly desert region.
They also say that in the last few months, Arab tribes in the area have been angered by executions by Iranian militias operating in the area of dozens of nomads suspected of affiliation with militants.
In December 2017, Iraq's then prime minister Haider Al Abadi announced a final victory against the ISIS. The group lost the last shred of its so-called caliphate in Syria in March 2019.
But sleeper cells have retained their ability to strike despite losing the territory they once held in both Iraq and Syria, operating mostly in the vast desert between the two countries.
In recent months Syria's Badia desert was the scene of regular clashes between the extremists and Russian-backed Syrian regime forces.
In April, 27 fighters loyal to the Damascus government and allied Iran-backed militiamen were killed in an ISIS attack near the desert city of Al Sukhna which is under regime control.
The war in Syria has killed more than 387,000 people since it started in 2011, the Observatory says.
The dead include more than 130,500 pro-government fighters, among them foreigners.
And since March 2019 more than 1,300 Syrian soldiers and allied pro-Iranian militiamen and over 600 IS fighters have been killed, according to the Observatory.