The Syrian Observatory for Human Rights said opposition forces attacked regime positions in the south of Idlib, killing at least 51 combatants on both sides, as Russian-backed regime forces have been making gains in the last opposition stronghold.
Clashes between anti-Assad fighters and regime forces killed over 50 combatants on both sides in northwestern Syria on Tuesday, a war monitor said.
Russia-backed regime forces have for weeks been chipping away at the edges of the opposition and rebel-run stronghold of Idlib — a province that borders Turkey — after bombarding it since late April.
But rebels on Tuesday attacked loyalist positions in the south of the bastion, the Britain-based Syrian Observatory for Human Rights said.
"Violent clashes east of the town of Khan Shaykhun broke out at dawn after jihadist and opposition groups attacked regime positions," Observatory chief Rami Abdel Rahman said.
The attack was led by the Al Qaeda-linked Hurras al Deen group and another faction Ansar al Deen, he said.
The fighting has killed 23 regime forces and 20 opponents, including 13 rebels, the Observatory said.
In the southeast of the bastion, eight rebels were killed trying to sneak through frontlines towards regime positions near the Abu Duhur military airport, the monitor added.
TRT World's Oubai Shahbandar has more details.
Regime forces recaptured Khan Shaykhun last week, and have been massing north of the town in recent days as they prepare to push on with their assault.
The town lies on a key highway running through Idlib province, and fully recapturing the artery would allow the regime to reconnect Damascus to second city Aleppo.
The heavy regime and Russian bombardment have hit areas north of Khan Shaykhun in recent days, in the vicinity of the town of Maarat al Numan, the next stop north on the highway.
On Monday, regime and Russian air strikes killed 12 civilians in the area, the Observatory reported.
The offensive comes despite a deal signed in September last year by Russia and Turkey to avert a full-blown assault on the Idlib region of some three million people.
The presidents of both countries met in Moscow on Tuesday.
The regime's escalation in bombardment since late April has killed more than 920 civilians, the Observatory says, and caused more than 400,000 people to flee their homes, according to the United Nations.
The Syrian civil war has killed more than 370,000 people and displaced millions since starting in 2011 with the brutal repression of anti-government protests.