The surprise offensive on Aleppo by Syrian rebel groups has left the US in the difficult position of not fully supporting either side while also maintaining a force posture of nearly 1,000 troops in Syria as part of the ongoing mission to fight ISIS.

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.

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.