Rebels who have inflicted big losses on Syrian President Bashar Assad say Russia's intervention in support of its ally will only lead to an escalation of the war and may encourage their Persian Gulf Arab backers to pour in more military aid.

Russia's deployment is prompting a reassessment of the conflict among insurgents whose advances in western Syria in recent months may have been the catalyst for Russia's decision. U.S. officials say Russian forces are already arriving.

Rebels say they have already encountered stronger government resistance in those areas — notably the coastal heartland of Assad's Alawite sect — and predict an even tougher war now that Russia is involved.