More than a dozen Syrian rebel factions have thrown their weight behind a proposed plan to create a single "national army" to unify the fractured opposition movement.

The idea was proposed earlier this week by the opposition's interim government in exile and by the Syrian Islamic Council (SIC), a body of Syrian Muslim clerics established in 2014 in Turkey.

"We must end the fractured state we are now facing and unify ranks," said the SIC, calling on rebels "to form one revolutionary army".

The interim government said the unified structure would "bring the downfall of the criminal regime".

By Friday, more than a dozen factions had backed the idea, including the prominent Islamist group Ahrar al-Sham and Turkish-backed rebels.

"We announce our support for the initiative launched by the SIC calling for a united national army," read a joint statement by Ankara-backed groups including the Mutasem Brigades.

Ahrar al-Sham said it was "ready to take all necessary steps to see the initiative succeed".

Since Syria's uprising broke out in March 2011, its various rebel components have tried and failed several times to form a lasting, unified front against President Bashar al-Assad.

In addition to losing territory to Assad's army, rebels have also clashed with jihadists from the Islamic State group and Hay'at Tahrir al-Sham.

That feeling of isolation may have pushed rebels to back this particular plan, said Charles Lister, an expert on Syria's rebel movement at the Middle East Institute.

"The only reason to say this might be different is because the opposition's situation is much more desperate than ever before," Lister told AFP on Friday.

"This is the only option they have left if they have any hope to be seen as legitimate."

Lister said idea was "very, very much a Turkish initiative" but had gained traction in recent weeks.

In particular, he added, rebels want to demonstrate to the international community that there were alternatives in Syria to Assad loyalists on one hand and jihadists on the other.