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.

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