At least 27 Syrian regime fighters were killed Thursday in an Islamic State group attack in the central province of Homs amid fierce clashes, a war monitor said.

Pro-government fighters backed by Russian air strikes were battling the jihadists to prevent them from entering the desert town of Al-Sukhna in Homs, the Syrian Observatory for Human Rights said.

The air raids and clashes killed 22 IS fighters, the Britain-based monitor said. "Russian aviation intervened to stop the jihadists from advancing and retaking the town," Observatory chief Rami Abdel Rahman said.

But the jihadists managed to break through and entered parts of Al-Sukhna in the evening, taking control of some neighbourhoods, he added. Syrian regime forces recaptured Al-Sukhna from IS in 2017.

Thursday's attack was the deadliest in the area since December, when IS fighters attacked an army garrison in a gas facility east of Homs city, killing four civilians and 13 troops or militiamen, Abdel Rahman said. IS proclaimed a "caliphate" in parts of Syria and neighbouring Iraq in 2014.

After years of various offensives against it, US-backed forces finally expelled IS from its last patch of territory in eastern Syria a year ago. But the jihadists retain a presence in the vast Badia desert stretching across the country through Homs province and eastwards to the Iraqi border, and they continue to carry out deadly attacks.

Syria's war has killed more than 380,000 people and displaced millions since starting in 2011 with the brutal repression of anti-government protests.