Hundreds of fighters from Iran -backed Iraqi militias have crossed into Syria to help the regime of Bashar al-Assad fight off an insurgency across much of the northwest of the country, which has seen the seizure of the second city of Aleppo.

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 Syrian military rushed reinforcements to the northwest and launched airstrikes Sunday in an attempt to push back insurgents who seized the country’s largest city of Aleppo, as Iran pledged to help the government counter the surprise offensive.