Clashes between anti-regime fighters and loyalist forces killed 51 combatants on both sides in northwestern Syria Tuesday, a war monitor said. Also on Tuesday, the opposition launched counterattacks in and near areas recently taken by regime forces in the extremist-run stronghold of Idlib.
Russia-backed regime fighters have for weeks been chipping away at the edges of the region after bombarding it for months.
But hardline opposition factions and extremists 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 Sheikhun broke out at dawn after extremist 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 extremist faction -- Ansar al-Deen -- he said.
The fighting has killed 23 regime forces and 20 opponents, including 13 extremists, the Observatory said.
In the southeast of the bastion, eight opposition fighters were killed trying to sneak through frontlines towards regime positions near the Abu Duhur military airport, the monitor added.
Regime forces recaptured Khan Sheikhoun 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.
Heavy regime and Russian bombardment has hit areas north of Khan Sheikhoun in recent days, in the vicinity of the town of Maaret al-Noman, the next stop north on the highway.
On Monday, regime and Russian airstrikes killed 12 civilians in the extremist stronghold, the Observatory reported.
The offensive comes despite a deal signed in September last year by Moscow and opposition-backer Ankara to avert a full-blown assault on the Idlib region of some three million people.
The presidents of both countries were set to meet in Moscow on Tuesday.
Hayat Tahrir al-Sham -- a group led by Syria's former Al-Qaeda affiliate -- extended its administrative control over the whole of Idlib in January, but other opposition factions remain present.
A spike 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 counterattacks began early in the morning and regime forces called in Syria's air force to repel them, the Observatory said.
The opposition captured two villages, Salloumieh and Abu Omar, and pushed into the nearby village of Sham al-Hawa, it said.
The Ibaa media outlet of Hayat Tahrir al-Sham said its fighters were attacking Syrian positions east of Khan Sheikhoun.
Elsewhere in northern Syria, a bomb exploded on a minibus, killing two people and wounding nine near the town of Azaz.
The town is controlled by Turkish troops and Turkey-backed opposition fighters, according to pro-regime media and the Azaz media center, an activist collective.
The Syrian civil war has killed more than 370,000 people and displaced millions since starting in 2011 with the brutal repression of anti-regime protests.
