UK-based Syrian Observatory for Human Rights says the Syrian regime wants to seize the southeast of Idlib province, which falls within a network of de-escalation zones where acts of aggression are prohibited.
Regime forces have retaken several villages in northwestern Syria, a monitor said on Thursday, a move that could tee up an offensive against the last major rebel bastion of Idlib.
More than six years into the deadly Syria conflict, Idlib province, which borders Turkey, is the only major region in the country still completely beyond the regime control.
Fierce clashes have in recent days pitted regime forces against Fateh al Sham Front, a former Al Qaeda affiliate, on the edge of the province, the Syrian Observatory for Human Rights said.
The monitor said regime air strikes in Idlib and Hama provinces killed 24 rebel fighters in the last 36 hours.
It said regime forces bombed opposition areas using more than 800 bombs and missiles in the last 24 hours.
"The army took several villages," the head of the Britain-based monitor, Rami Abdel Rahman, said.
He said the regime push was backed by Russian air strikes and added that the "regime wants to seize the southeast of Idlib province."
The latest fighting took place in villages on the border between Idlib and Hama provinces where clashes have been ongoing for two months.
A Syrian local civil defence agency, the White Helmets, also said Russian and Syrian warplanes repeatedly struck residential areas in the town of Al Lataminah and six villages, killing at least 22 people.
Idlib falls within a network of de-escalation zones - endorsed by Turkey, Russia and Iran - in which acts of aggression are expressly prohibited.
Syria has begun to emerge from a devastating civil war that began in early 2011 when the Assad regime cracked down on pro-democracy protests.
Hundreds of thousands have been killed and more than 10 million forced to flee their homes in the conflict, the UN has said.