Why ISIS Is Gaining Ground – and So Hard to Beat

'The Syrian opposition' here means the civilian population Assad is bombing, and so is at one with 'rebels' in needing to get rid of him.
"The rebels can’t divert resources from the fight against the regime to go fight ISIS. By doing so they would enable the regime to make additional gains, which in the case of Aleppo would be particularly devastating for the rebel militants and the Syrian opposition in general."