Syrian government forces and their Russian allies are battling to hold back the offensive, with Russia launching airstrikes on Aleppo and the western city of Idlib on Sunday as Syrian government forces regroup, according to monitoring group The Syrian Observatory for Human Rights.

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.

Anti-government fighters are continuing to advance even as Syrian and Russian forces seek to halt their progress.