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

The Syrian army said on Saturday dozens of its soldiers had been killed in a major attack led by Islamist Hayat Tahrir al-Sham rebels who swept into the city of Aleppo, forcing the army to redeploy in the biggest challenge to President Bashar al-Assad in years.

Over a few days, rebel fighters in northern Syria have launched incursions into several major cities with little resistance from government troops.