An attack by Islamic State group jihadists on Syrian government forces in the war-torn country’s east has killed 33 soldiers, a monitor said Saturday, revising an earlier toll of 26 deaths.

The shooting Thursday evening on an army bus was the extremist group’s deadliest attack on government forces this year, according to the Syrian Observatory for Human Rights.

Despite losing their last piece of territory in Syria in 2019, IS 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.