Syria’s rebels seized most of Aleppo on Saturday as their sweeping advance ousted Russian forces from at least three military bases around the city.

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.

The Syrian army has said 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 army to redeploy in the biggest challenge to President Bashar al-Assad in years.