Jihadist attack kills 30 soldiers in east Syria: monitor
"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.
The official news agency SANA reported that a "terrorist attack" on a bus killed "25 citizens" and wounded 13.
Abdel Rahman said the bus was ambushed near the village of Shula by jihadists who detonated bombs before opening fire.
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 defeated in Syria in March last year but sleeper cells continue to launch attacks namely in Syria's vast desert that stretches from the central province of Homs to Deir Ezzor and the border with Iraq.
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.
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.