On Wednesday, HTS and its allies said they had launched their offensive to “deter aggression” and “thwart the enemy’s plans”, accusing the government and allied Iran-backed militias of escalation and aggression in north-west.
But it came as the Syrian government and its allies were preoccupied with other conflicts.
In neighbouring Lebanon, an Israeli military campaign has devastated the Iran-backed Hezbollah movement, whose fighters helped turned the tide of the Syrian civil war.
Israel has also stepped up its air strikes inside Syria on targets linked to Iran, Hezbollah and other Iran-backed militia groups, while Russian forces are focused on the war in Ukraine.
By the end of the first day of the offensive, the rebels had advanced into the western Aleppo countryside, taking them within 10km (6 miles) from the outskirts of Aleppo city, according to the UK-based Syrian Observatory for Human Rights (SOHR).
It reported that they had seized the Syrian army 46th Regiment’s base and at least eight villages.
On Thursday, the monitoring group said the rebels had cut the M5 highway between Aleppo and the capital Damascus near Zarbah, 15km south-west of Aleppo city, and taken control of the interchange between the M5 and the M4 highway further south, near Saraqeb.
The SOHR said 121 rebels, most of them members of HTS, and 40 government troops and 21 militiamen had been killed over the past two days.
The rebels said in a Telegram statement that they had seized the town of Khan al-Assal, which is 5km west of Aleppo city, and had killed more than 200 members of pro-government forces.
A Syrian military statement put out on Thursday said its forces had “confronted the terrorist attack that is still ongoing with various weapons and in co-operation with friendly forces, leaving heavy losses in equipment and causalities among terrorists”.
It did mention any losses among its forces, but Iranian news agencies said a senior commander in Iran’s Revolutionary Guards serving as a “military adviser” in Syria, Brig Gen Kioumars Pourhashemi, had been killed in Aleppo province.