IDLIB: Two hardline Syrian rebel factions announced they were merging on Sunday, in an apparent attempt to pre-empt a broader clash with jihadists in the country's fractured northwest.

"We, Ahrar al-Sham and Nureddine al-Zinki, announce our merger under the Syrian Liberation Front," the two groups announced.

Both groups are hardline Islamist rebel factions with a strong presence in rebel-held parts of the country's north, along the border between the provinces of Aleppo and Idlib.

Idlib province is held mostly by Hayat Tahrir al-Sham (HTS), an alliance dominated by jihadists once linked to Al-Qaeda.

In their founding statement, the Syrian Liberation Front called on rebel groups to join their new movement.

Sam Heller, a senior analyst at the International Crisis Group, said the merger was linked to HTS's growing power in northern Syria.

"This is an attempt to form a counterweight to Hayat Tahrir al-Sham, which is apparently positioning itself for a new fight with Zinki," he told AFP.

"Zinki and Ahrar are the two most substantial non-jihadist factions in the rebel-held north, and Zinki in particular has been an obstacle to Hayat Tahrir al-Sham's attempts to consolidate control."

Syria's conflict erupted in 2011 with protests against the government of Bashar al-Assad, but it has since evolved into a complex war.

The opposition movement has long been fractured, but mainstream rebels have struggled in recent years to fight off the influence of powerful jihadist factions like HTS.