Syrian and Russian jets are stepping up strikes on opposition forces in northern Syria in retaliation for the sudden offensive that has cost the autocratic regime control of the country’s second largest city,

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.

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.