Clashes between anti-government fighters and regime forces killed 51 combattants on both sides in northwestern Syria Tuesday, a war monitor said.

Russia-backed regime fighters have for weeks been chipping away at the edges of the militant-run stronghold of Idlib — a province that borders Turkey — after bombarding it for months.

But hardline 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 Sheikhun broke out at dawn after militants 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 militant faction — Ansar al-Deen -- he said.

The fighting has killed 23 regime forces and 20 opponents, including 13 militants, 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.

Regime forces recaptured Khan Sheikhun 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 government to reconnect Damascus to second city Aleppo.

Heavy regime and Russian bombardment has hit areas north of Khan Sheikhun 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 air strikes killed 12 civilians in the militant stronghold, the Observatory reported.

The offensive comes despite a deal signed in September last year by Moscow and rebel 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 rebel 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 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.