Several civilians and tens of rebels have been killed in several air strikes since the early morning, according to the Syrian Observatory for Human Rights (SOHR).
It was the first time air raids had targeted Aleppo since 2016, when the Syrian opposition was driven out of the city.
However, rebels led by Hay’at Tahrir al-Sham (HTS) and allied groups, including some backed by Turkey, claimed stunning gains on Saturday.
They claimed to have seized Aleppo International Airport and the strategic city of Khan Sheikhoun in southern Idlib.
The administrative borders of Idlib Governorate were fully under their control, they added.
They also claimed to have begun marching towards Hama, successfully capturing six towns and villages in the countryside, including Morek, which lies along an important highway connecting central Syria to the north.
The offensive began on Wednesday when rebels broke out from opposition-held territory in northwest Syria towards Aleppo.
Within two days, they had seized dozens of towns and villages, as well as a section of the strategic M5 highway, cutting off supply routes to Damascus.
They have taken several military bases and fortified positions since, often meeting little resistance.
The Syrian government acknowledged the rebels’ advances on Saturday.
It announced its forces were carrying out a “redeployment operation” to strengthen its defences, “absorb the attack, preserve the lives of civilians and soldiers, and prepare for a counterattack”.
It also took responsibility for some of the air strikes on Aleppo, saying they aimed to prevent rebels from establishing fixed positions.
According to SOHR, government forces have collapsed in Idlib and Aleppo.
This has left Aleppo, Syria’s second-largest city, outside government control for the first time since the country’s independence in 1946, the monitoring group said.
Nearly 330 people have been killed since the offensive began, mostly fighters on both sides, according to SOHR.
Amid fast-moving developments, the foreign ministers of Turkey and Russia – both major stakeholders in Syria – spoke by phone on Saturday and agreed to coordinate efforts to stabilise Syria, according to Moscow.
“Both sides expressed serious concerns at the dangerous development of the situation in the Syrian Arab Republic in connection with the military escalation in the Aleppo and Idlib provinces,” the Russian ministry said.
Russian Foreign Minister Sergey Lavrov also spoke by phone to his Iranian counterpart, according to Iranian state media.
The frontlines of Syria’s war have barely shifted since 2020. A “de-escalation” agreement in 2019 between rebel-backer Turkey and Syria’s President Bashar al-Assad’s sponsors, Russia and Iran, had created some stability and a long-term ceasefire.
Most of Idlib province has since been held by HTS, a former al-Qaeda affiliate, which has established a civilian administration.
Turkey-backed rebel groups in the Syrian National Army coalition have held sway in other areas of the north.
However, despite Russia being distracted by the war in Ukraine and Assad’s forces weakened by frequent Israeli attacks, Syrian and Russian warplanes have stepped up air attacks on opposition-held areas since August 2023.
Meanwhile, Assad’s government used the stability to make diplomatic inroads, normalising relations with several regional countries and rejoining the Arab League.
That stability now appears severely undermined. Aleppo became an opposition stronghold after the revolution broke out in 2011. Its 2016 capture by Assad’s forces was highly symbolic.
