The group of 41 rebel factions, including the powerful Islamist groups Ahrar al-Sham, Islam Army and the Levant Front said Russia and Iran were occupying forces and should be attacked.

"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," the factions said a statement. "Any occupation force to our beloved country is a legitimate target."

The statement of unity from an otherwise fractured Syrian opposition comes as Russia continues to bomb what it says are targets belonging to "Islamic State" group and al-Qaeda's Syrian branch, al-Nusra. On Monday, the Russian defense ministry said its planes hit nine "Islamic State" targets in the past twenty-four hours.

Russian airstrikes have also reportedly hit Ahrar al-Sham and other groups dubbed by Moscow and Syria as terrorist organizations.

International concern

The United States and its allies have voiced concern the airstrikes since last Wednesday have largely targeted "moderate" rebel groups in central and northwestern Syria in a bid to prop up the Assad regime.

US Defense Secretary Ash Carter said on Monday that Russia was pursuing a "losing strategy" to support Assad and the airstrikes would only exacerbate problems.

"Russia has escalated the civil war, putting further at risk the very political resolution and preservation of Syria's structure of future governance it says that it wants," Carter said in Madrid as he began a five-day European tour.

On Monday, Germany also expressed concern Moscow's airstrikes were targeting moderate rebels.

The rebel statement, which called for regional states to unite behind the rebel factions, did not include al-Nusra, which is part of a larger umbrella rebel group, the Army of Conquest. The Army of Conquest also includes Ahrar al-Sham, which up until the Russian airstrikes had made advances against regime forces.

The rebel groups have largely been backed by Qatar, Saudi Arabia and Turkey.

Calls for jihad

In a sign that sectarianism is further inflaming the region, a group of Saudi clerics on Monday called for jihad against Assad's Alawite regime, Iran and Russia.

In a statement tinged with sectarian references, 53 Saudi clerics said Sunni Muslim states had an obligation to help defend Islamists in Syria.

"The holy warriors of Syria are defending the whole Islamic nation. Trust them and support them ... because if they are defeated, God forbid, it will be the turn of one Sunni country after another," the clerics, who are not tied to the government, said.

"The Western-Russian coalition with the Safavids (Iran) and the Nusairis (Alawites) are making a real war against the Sunni people and their countries," the statement said.

Meanwhile, Syria's Muslim Brotherhood on Monday also called for jihad against Russian occupation, according to militant websites. The Brotherhood stated Syria is "witnessing a sheer and clear Russian occupation" and resistance was legitimate.

The Russian intervention in Syria has stoked concerns it could further radicalize Syrian Islamist groups and "moderate" factions and lead to a new wave of foreign fighters entering the country to wage jihad.

The Soviet Union's invasion of Afghanistan in the 1980s as well as the war in Chechnya provoked a global jihadist movement drawing thousands of fighters.

cw/se (AP, Reuters)