This file frame grab from video shows a Daesh fighter firing his weapon in Baghouz, Syria. File/AP

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

The shooting on Thursday evening on an army bus was the militant 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, Daesh 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 militants surrounded the bus in the desert near Mayadeen, in Deir Ezzor province, and opened fire, the Observatory reported on Friday.

Daesh 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 militants' Amaq news agency.

Agence Frnace-Presse