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.

Syrian rebels have swept through parts of the country at lightning pace, taking control of the the second-largest city, Aleppo. But who are they and what are their aims?

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