Syrian rebels advanced to within 10 kilometers (six miles) of the northern city of Aleppo on Wednesday, according to a war monitor, constituting a major escalation in the ongoing civil war.

The UK-based Syrian Observatory for Human Rights reported that Hayat Tahrir al-Sham (HTS) and the Syrian National Army captured 21 villages in the western Aleppo countryside. A site belonging to the 46th regiment of government forces, located near Atarib, was among the areas they took. At least 36 HTS members and 16 Syrian National Army fighters were killed, in addition to 37 killed among the government forces. Five government soldiers were also captured, according to the observatory.

The observatory and the pro-opposition Syria TV reported heavy Syrian and Russian bombing in the area on Wednesday.

Hassan Abdul Ghani, a spokesperson for the Al-Fatah al-Mubin coalition of HTS and other rebel groups, announced the “start of the deterrence operation to protect the people of the liberated areas Assad’s gangs and the Syrian militias” in a post on X on Wednesday, referring to forces loyal to Syrian President Bashar al-Assad.