BEIRUT (AFP) - Clashes between Russia-backed Syrian regime forces and the Islamic State in Iraq and Syria (ISIS) group have killed more than 50 fighters on both sides in two days, a Britain-based war monitor said on Saturday (July 4).

Fighting and Russian air strikes in the central desert province of Homs since late on Thursday have claimed the lives of 20 pro-government fighters and 31 militants, the Syrian Observatory for Human Rights said.

"The fighting started in the night of Thursday to Friday with a jihadist assault on regime positions" near the town of Al-Sukhna, Observatory head Rami Abdel Rahman said.

ISIS militants have retained a roving presence in Syria's vast Badia desert, despite losing their last shred of territory last year.

They regularly carry out attacks there.

ISIS declared a cross-border "caliphate" in large parts of Syria and neighbouring Iraq in 2014, but several military campaigns against it chipped away at that proto-state and eventually led to its territorial demise.

Syria's war has killed more than 380,000 people since it started in 2011 with the repression of anti-government protests, before evolving into a complex conflict involving world powers and militants.