Free Syrian Army
Rebel fighters from the "First Battalion" under the Free Syrian Army are shown taking part in military training in June in the rebel-held countryside of the northern city of Aleppo. A Russian official said Monday that Russian forces were ready to cooperate with the anti-Assad militant group in order to fight ISIS. AFP/Getty Images

As many as 41 rebel groups in Syria, considered fragmented in general, have vowed to come together against Russian forces which performed an air campaign.

The groups include major insurgent organizations like Islam Army, the Levant Front and Ahrar al-Sham. According to the groups, Syrian President Bashar Assad was on the verge of “a crushing defeat” when Russia joined the war.

Russia started its air campaign Wednesday after having claimed to target Islamic State group and al Nusra militants. However, the Associated Press reported that a number of strikes apparently targeted Western-backed rebel factions.

Meanwhile, NATO has called Russia’s military actions in Syria as “dangerous.” It said in a statement that allies are concerned about the Russian military build-up in Syria. The allies are especially worried about “the attacks by the Russian Air Force on Hama, Homs, and Idlib which led to civilian casualties” and did not target ISIS.

NATO allies have asked Russia to put an immediate end to its attacks on civilians and the Syrian opposition. They have also asked Russia to focus on fighting ISIS militants and promote a political transition to reach a solution to the ongoing crisis.

“Russian military actions have reached a more dangerous level with the recent violations of Turkish airspace on 3 October and 4 October by Russian Air Force SU-30 and SU-24 aircraft in the Hatay region,” the NATO statement said. “The aircraft in question entered Turkish airspace despite Turkish authorities’ clear, timely and repeated warnings.”

According to U.K.-based Syrian Observatory for Human rights, Russian airstrikes allegedly left a large number of casualties after warplanes had targeted the northern town of Al-Bab, a stronghold of the ISIS.