A faction linked to Hayat Tahrir al-Sham "early Saturday attacked checkpoints and positions of regime forces on the western outskirts of Aleppo city", the Syrian Observatory for Human Rights said.
Heavy fighting and shelling broke out between both sides, so far "killing at least 13 regime troops and allies fighters", the Britain-based monitoring group said.
Army shelling kills seven in Syria's Idlib: monitor
The ongoing clashes have also taken the lives of at least eight members of the Abu Bakr Al-Sadeeq Army militant faction, Observatory chief Rami Abdel Rahman added.
Militant factions control the countryside to the west of Aleppo, which is part of the broader Idlib region under administrative control of HTS.
That region is in theory protected from a massive regime military offensive by a deal inked in September by government ally Russia and rebel backer Turkey.
But the accord has unwound as HTS took full control of the region in January after defeating rival rebels, and the area has come under increasing bombardment.
The Observatory said on Saturday's attack came after regime bombardment overnight hit the western part of Aleppo province and the southeastern part of Idlib province.
A planned buffer zone around the region was never fully implemented as militants refused to withdraw from it.
President Bashar al-Assad's troops have managed to claw back some two thirds of the country since Russia intervened militarily in 2015, but Idlib remains beyond regime control.
On Friday, Assad insisted the main aim of the Idlib deal was to "eliminate terrorists", after they failed to pull out from the planned demilitarised area.
He urged progress on removing "obstacles" to the stalled deal ahead of talks to be attended by Russia, fellow ally Iran, and Turkey next week in Kazakhstan.
Assad urges progress on Idlib deal ahead of Syria talks
Saturday's attack comes after the Observatory reported that the Islamic State group had killed at least 35 regime fighters in 48 hours in the country's Badia desert further south.
They were the deadliest attacks since US-backed forces announced the defeat of that militant group's "caliphate" on the border with Iraq last month.
Syria's war has killed more than 370,000 people and displaced millions since the conflict began with the repression of anti-government protests in 2011.
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