The insurgent offensive, shocking for its speed, has posed the most serious challenge to President Bashar al-Assad in years.

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.

Syrian opposition forces have taken control of much of the country’s second largest city Aleppo after a lightening advance that killed dozens of government soldiers in a major challenge to President Bashar al-Assad.