The civil war has largely quieted down since a ceasefire was agreed in 2020, but rebels still control the northwestern city of Idlib and most of the surrounding area.
Idlib is located just 55 kilometers (34 miles) from Aleppo, which was a rebel stronghold until it fell to government forces in 2016.
The offensive, which began on Wednesday, is being led by the jihadist group known as Hayat Tahrir al-Sham (HTS) and its Turkish-backed allies.
HTS was considered one of the most effective and lethal groups fighting the Assad government and was already a dominant force in Idlib.
According to SOHR, rebels have taken control of Aleppo airport and dozens of nearby villages.
They also announced a curfew that will come into effect from 17:00 local time (14:00 GMT) and will remain in place until Sunday.
The Syrian army said the rebels “launched widespread attacks using multiple axes on the Aleppo and Idlib front lines,” and that “fighting took place in an area of over 100 km.”
It said dozens of soldiers had been killed and admitted rebels had entered “large parts” of Aleppo.
Government forces said they had temporarily withdrawn from Aleppo, but added that they were preparing a counterattack.
Photos showed roads leading into Aleppo filled with cars on Saturday as residents tried to leave.
