Hayat Tahrir al-Sham (HTS), a jihadist group led by Syria's former al-Qaeda affiliate, has since January controlled most of Idlib province as well as parts of neighbouring Hama, Aleppo and Latakia provinces.
Several other armed rebel groups also operate in the region.
Fighting in southern Idlib and rural Latakia on Tuesday claimed the lives of 29 pro-government forces as well as 30 jihadists and allied rebels, the Syrian Observatory for Human Rights said.
It came as regime warplanes pummelled the Idlib region with air strikes, killing three civilians in the town of Khan Sheikhoun, the Britain-based monitor added.
Regime forces and anti-government fighters have been caught in fierce battles in the region for days, as the former presses with an advance toward a strip straddling the Hama and Idlib governorates.
On Sunday, regime forces seized the town of Al-Habeet in Idlib's southern countryside, in their first major ground advance in the province since an escalation on the jihadist-dominated enclave more than three months ago.
The region was supposed to be protected from a massive government offensive by a Turkish-Russian buffer zone deal struck last September.
But it has come under increasing bombardment by Damascus and its backer Moscow since the end of April that has killed 816 civilians, according to the Observatory.
The violence has also pushed 400,000 people from their homes, according to the United Nations.
Syria's conflict has killed a total of more than 370,000 people and displaced millions since it started with the brutal repression of anti-government protests in 2011.
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