The surprise attack, led by the Islamist group Hayat Tahrir al-Sham, was the boldest rebel assault for years in the civil war that has been frozen.

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 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.

Russia has promised Damascus additional military assistance that is expected to arrive in the next 72 hours, as rebel forces sweep the northwestern city of Aleppo, two sources in the Syrian military told Reuters.