Syrian rebels opposed to President Bashar al-Assad said on Friday they had reached the heart of the northern city of Aleppo, after a surprise sweep through government-held towns and nearly a decade after having been forced out of the city.

BEIRUT—Syria’s antigovernment rebels claimed a stunning advance into Aleppo, a major symbolic and strategic prize, in a setback for the government in Damascus and its allies Russia and Iran.Islamist-led insurgents poured into Aleppo two days after launching a stunning strike from the country’s rebel-held northwest and eight years after the city fell to forces loyal to the government of Bashar al-Assad at the end of a long siege.

Syrian rebels launched a surprise offensive into western Aleppo, challenging years of government control and reigniting fears in a war-weary city.