At least 27 Syrian regime fighters were killed in an ISIS attack in the central province of Homs on Thursday, a war monitor said.
Pro-government fighters backed by Russian air strikes were battling the militants 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 ISIS 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 militants managed to break through and entered parts of Al Sukhna in the evening, taking control of some neighbourhoods, he said.
Syrian regime forces recaptured Al Sukhna from ISIS in 2017.
Thursday's attack was the deadliest in the area since December, when ISIS fighters attacked an army garrison in a gas facility east of Homs city, killing four civilians and 13 troops or militiamen, Mr Abdel Rahman said.
ISIS proclaimed a "caliphate" in parts of Syria and neighbouring Iraq in 2014.
After years of various offensives against it, US-backed forces finally expelled ISIS from its last patch of territory in eastern Syria a year ago.
But the extremist group retains 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.