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

The rapid advance on Aleppo came just four days into a surprise opposition offensive that is the most intense escalation in years in the civil war.

Syria’s antigovernment forces claimed a stunning advance into Aleppo, in a setback for the government in Damascus and its allies Russia and Iran.