Russian military support crew attach a satellite guided bomb to SU-34 jet fighter at Hmeimim airbase in Syria.
Beirut - Syria's Muslim Brotherhood declared that fight against the "sheer Russian occupation of Syria" is a legitimate duty for everyone.
More than 40 Syrian insurgent groups vowed to attack Russian forces in retaliation for Moscow's air campaign in a show of unity among the usually fragmented rebels against what they called the "occupiers" of Syria.
The 41 groups, which included powerful factions such as Ahrar Al Sham, Islam Army and the Levant Front, said Russia joined the war in Syria after President Bashar Assad's forces were on the verge "of a crushing defeat."
Russia launched its air campaign on Wednesday and claims it's targeting the Daesh group and Al Qaeda's Syrian affiliate, the Nusra Front. But many of the strikes appear to have hit Western-backed rebel factions.
The Russian attacks have largely focused on the northwestern and central provinces - the gateways to the heartland of Assad's powerbase in the capital, Damascus, and on the Mediterranean coast.
The Russian intervention was widely criticised by Syrian opposition groups and activists especially since Moscow once played the role of a mediator hosting rounds of talks between the Syrian government and its opponents.
"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 41 factions said in a statement released by Ahrar Al Sham. It was apparently referring to backers of the opposition such as Turkey and Saudi Arabia.
A Syrian military official was quoted by state media as saying that Russian airstrikes on Monday hit in the central province of Homs and Idlib in the northwest.
The Britain-based Syrian Observatory for Human rights said warplanes believed to be Russian have targeted the northern town of Al Bab that is a stronghold of the Daesh group. It said the airstrike left "a large number" of casualties.
A Facebook page used by Daesh posted photos of wounded people being treated in a clinic and another of what appeared to be a burned body being pulled out of a charred car. The Facebook page said dozens of people were killed or wounded in the airstrikes.
The Syrian militant and rebel factions, including the US-backed Division 101 and Tajammu Alezza, said "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."
"All Syrian armed revolutionary factions must realise we are in a war to push an aggressor, a war that makes unifying ranks and word a duty on all brothers," the factions said in the two-page statement posted online. "Any occupation force to our beloved country is a legitimate target."
Earlier in the day militant websites reported that Syria's Muslim Brotherhood declared that fight against the "sheer Russian occupation of Syria" is a legitimate duty for everyone capable of carrying weapons.
The Brotherhood's media chief Omar Mushawah was quoted as saying that "we as Muslim Brotherhood group confirm that we are witnessing a sheer and clear Russian occupation" and that "resisting the occupier is a legitimate duty."
Meanwhile, Syria's foreign minister said Russian airstrikes and its new military "steps" in Syria took months of preparations.
Walid Al Moallem said in remarks broadcast on Beirut's Al Mayadeen TV that Russia closely coordinates with the Syrian army over its airstrikes. The full interview with Al Moallem will be aired later on Monday.
In neighbouring Lebanon, a roadside bomb hit a minibus carrying passengers in the eastern town of Chtoura near the Syrian border without causing any casualties, the Lebanese army said.
Some local media reports said the bus belonged to Hezbollah while it was on its way to Syria, where the group has been fighting in alliance with Assad's forces.