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 an unexpected 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.

Insurgents have breached Syria’s second-largest city, Aleppo, clashing with government forces, according to a Syria war monitor and fighters.

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