In an unprecedented show of unity, more than 40 Syrian rebel factions issued a joint statement condemning Russia's intervention in the ongoing civil war and vowing to fight Russian "occupiers."
Russia claims it launched its massive air campaign last week to target the jihadists ISIS terror group, as well as Al Qaeda's Syrian franchise, Al Nusra Front.
However, while Russian jets have struck ISIS positions in their northern stronghold of Al-Bab - which the London-based Syrian Observatory for Human Rights says resulted in "a large number" of casualties - the vast majority of strikes have been directed against other rebel groups seeking to topple embattled dictator Bashar al-Assad, including several US-backed groups involved in fighting ISIS.
The unabashed use of Russian military force also includes a large number of ground forces, deployed in the Syrian regime's western stronghold to fend off advancing rebels. It is being seen as yet another sign of how the US administration's hesitation in Syria has opened the way for other actors such as Iran and Russia to prop up the Assad regime, while advancing their own interests in the region.
But the Russian intervention could also backfire, pushing a fractious Syrian rebel movement into a cohesive front against the emerging threat.
Rebels in the north and south of Syria have already largely coalesced into two major rebel alliances, but divisions still remain.
But in a statement issued by 41 rebel groups - including the powerful Ahrar al-Sham, which released the statement - a wide variety of disparate rebel brigades called for a new "regional alliance" against what they branded as a "Russian-Iranian occupation" of Syria.
The lengthy statement was also signed by smaller, US-backed groups, including Division 101 and Tajammu Alezza.
"This new reality requires the region’s countries and the allies in specific to hasten in forming a regional alliance to face the Russian-Iranian alliance that occupies Syria," the statement read.
"The Russian military aggression on Syria is considered a blatant occupation of the country even if some claim it was done with the official request of the Assad regime. Those who lost legitimacy can’t offer it," it continued.
"All Syrian armed revolutionary factions must realize we are in a war to push an aggressor, a war that makes unifying ranks and word a duty on all brothers. Any occupation force to our beloved country is a legitimate target."
And in comments reported by AP, Syria's Muslim Brotherhood joined the chorus, calling for a "jihad" against Russia.
The Brotherhood’s chief spokesman Omar Mushawah was quoted on Islamist websites urging all those capable of carrying weapons to fight the "sheer Russian occupation."
"We as Muslim Brotherhood group confirm that we are witnessing a sheer and clear Russian occupation," he said, adding that "resisting the occupier is a legitimate duty."