ISIS militants attacked Syrian government forces and their allies in countryside east of Homs and Hama on Wednesday, the Syrian Observatory for Human Rights said.
Heavy fighting raged around the mountainous Jabal al-Shomariya area in Homs province, and the government side was carrying out air strikes there, the British-based monitoring group said.
Pro-Damascus media outlets quoted a military source saying warplanes had hit targets in the eastern Hama countryside, which borders Jabal al-Shomariya to the north.
ISIS has been losing ground to government forces further east, close to its stronghold of Deir al-Zor province and al-Sukhna, the last town it holds in Homs province.
Several sides in Syria’s crowded battlefield are fighting ISIS, including Syrian government forces backed by Russia and Iran, and US-backed, Kurdish-dominated forces around its Raqqa stronghold.