Islamic State group jihadists killed 37 soldiers when they ambushed a bus Wednesday in one of the deadliest attacks since the fall of their “caliphate” last year, a monitor said.

The Syrian Observatory for Human Rights said the attack in the eastern province of Deir Ezzor targeted the regime soldiers as they travelled home for holidays.

The official news agency SANA reported that a “terrorist attack” on a bus killed “25 citizens” and wounded 13.

IS 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 namely in the vast desert that stretches from the central province of Homs to Deir Ezzor and the border with Iraq.

“It was one of the deadliest attacks since the fall of the IS (self-proclaimed) caliphate” last year, Observatory head Rami Abdel Rahman told AFP, referring to Wednesday’s bus ambush.

Abdel Rahman said the bus was attacked near the village of Shula by jihadists who detonated roadside bombs before opening fire on the soldiers who belonged to the army’s 4th division.

At least 37 soldiers were killed, including eight officers, and 12 others wounded, the war monitor said in an updated toll, adding that some of those injured were in “critical condition”.

Two other buses which were part of the convoy managed to escape, the Observatory said.

There was no immediate claim of responsibility for the attack.

In December 2017, Iraq’s then prime minister Haider al-Abadi announced a final victory against the IS and the group lost the last shred of their 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 jihadists and Russian-backed Syrian regime forces.

In April, 27 fighters loyal to the Damascus government and allied Iran-backed militiamen were killed in an IS 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.