Once known as the Al Nusra Front, the Hayat Tahrir al-Sham (HTS) has advanced further south towards the city of Hama after capturing Aleppo. The Syrian army has withdrawn temporarily from the area to redeploy for a counter attack.
The rebels were spotted at a central square in Aleppo as well as its citadel. They also claimed control of the airport, which is 7 km from the city.
The Kurdish militia, which is known as YPG, also extended cotrol of a few neighbourhoods in Aleppo following the withdrawal of the army.
The Bashar al-Assad government's forces now control only a single area in Aleppo, The Washington Post quoted opposition activist Abdulkali Alhamdo as saying.
This comes as the Russian air force on Sunday carried out airstrikes to support the Syrian army. The strikes targeted the rebels who took control of rural Idlib and Hama.
HTS had announced earlier that it captured the outskirts of Hama while Syrian army vehicles were spotted in the northern part of the city, indicating the front of war between the two sides.
The previous Russian strikes on Aleppo was in 2016, when the Moscow-allied Assad regime took control of the city from the rebels.
The US said it had nothing to do with the rebel offensive, claiming that Assad's reliance on Moscow and Tehran led to the fresh offensive. It added that the US considers the HTS a "terrorist" outfit.
The Syrian army said dozens of its soldiers have been killed since the offensive began. The army has withdrawn in order to regroup and redeploy troops to bolster its frontlines for a counterattack.
Civil defence group, the White Helmets, revealed that 31 civilians, including 10 children, were killed in the attacks by the Assad regime while 113 others were wounded.
Meanwhile, Iranian foreign minister Abbas Araghchi is set to travel to Damascus to discuss strategies with the Assad regime.