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.

Russia and Iran have offered military and diplomatic support to the Syrian government as it tries to stanch a major rebel advance.