The Islamic State group claimed responsibility Thursday for an attack that killed nearly 40 soldiers in Syria the day before when jihadists ambushed a bus in eastern Syria.

War monitor the Syrian Observatory for Human Rights said on Wednesday IS had attacked regime soldiers as they travelled home for holidays in the eastern province of Deir Ezzor, killing 37.

It said eight officers were among those killed while 12 other soldiers were wounded in the attack.

A statement by IS's propaganda arm Amaq said its fighters had "ambushed a bus transporting apostate Nusayri army elements," using a derogatory term for the Alawite sect to which President Bashar al-Assad belongs.

The vehicle was targeted "with heavy weapons" and "multiple explosive devices, which led to destroying the bus and killing nearly 40 elements and wounding others", added the statement, according to SITE Intelligence, which monitors jihadist activities worldwide. 

"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 on Wednesday.

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.

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

The Observatory said two other buses which were part of the convoy managed to escape.

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, as well as 117,000 civilians.