Syrian rebels, led by U.S. designated terrorist group Hayat Tahrir al-Sham, have taken control of most of Syria’s second-largest city, Aleppo, after launching a surprise attack against President Bashar al-Assad’s regime on Wednesday.

Apparently unprepared for the attack, Assad’s army retreated and offered little resistance as the Turkey-backed rebels took the majority of the city. The regime claimed the rebels were unable to establish “solid” positions, and that they are regrouping in preparation for a counteroffensive.