Despite losing their last piece of territory in Syria in 2019, ISIS has maintained hideouts in the vast Syrian desert from which it has carried out ambushes and hit-and-run attacks.

“The death toll from the army bus attack rose to 33 soldiers,” said Rami Abdel Rahman, who heads the British-based monitoring group which relies on a wide network of sources inside Syria.

The jihadists surrounded the bus in the desert near Mayadeen, in Deir Ezzor province, and opened fire, the Observatory reported on Friday.

IS claimed the attack later Friday, saying its fighters had carried out an ambush “on two military buses”, targeting them “with heavy weapons and rocket-propelled grenades” and setting one on fire, according to a statement from the jihadists’ Amaq news agency.

Syrian state news agency SANA said the “terrorist attack” had caused a number of military casualties, citing an army source.

Abdel Rahman said ISIS “has recently been escalating its deadly military attacks... aiming to cause as many deaths as possible”.

By doing so, the group are trying to show that ISIS “is still active and powerful despite the targeting of its leaders”, he told AFP.

Last week, ISIS announced the death of its leader Abu al-Hussein al-Husseini al-Qurashi, who it said was killed in clashes in northwestern Syria, and named a successor.

ISIS members in recent weeks have increased their attacks in Syria’s north and northeast.

Earlier this week, 10 Syrian soldiers and pro-government fighters were killed in an ISIS attack in the Raqa province, the Observatory said.

Syria’s war broke after President Bashar al-Assad’s government crushed peaceful protests in 2011. It has since drawn in foreign powers and global jihadists.

The conflict has killed more than half a million people and driven half of the country’s pre-war population from their homes.