Russia has conducted major air strikes on rebels in Syria after insurgents led by the Islamist Hayat Tahrir al-Sham group swept into Aleppo in what is the biggest challenge to its ally President Bashar al-Assad in years.

In just a few days, a patchwork of rebel forces has seized control of a broad stretch of land in the west and northwest of Syria. But it’s unclear just how much support Syria’s weakened allies can provide.