AMMAN (Reuters) - The main Turkish-backed rebel army has deployed along frontlines close to jihadists in northwest Syria to repel any new militant advance after an offensive that expanded their control over the country’s last rebel-held bastion, rebels and residents said.

Islamist Tahrir al Sham, formerly affiliated to al Qaeda, on Sunday entered the town of Atareb, days after capturing from their mainstream rivals the strategic town of Darat Izza, both in the western Aleppo countryside, in a military campaign that has since spread across Idlib and areas near the Turkish border.

A convoy of Tahrir al Sham fighters entered the densely populated town after forcing its leaders to hand over control by threatening to storm it if mainstream rebels opposed to their hardline Islamist ideology do not leave.

The offensive by Tahrir al Sham has alarmed the National Army, the main Turkish-backed rebel force aimed at unifying disparate factions in the northwest.

“We took a decision to repel the aggression by Tahrir al Sham on the Aleppo and Idlib countryside,” their spokesman Major Youssef Hamoud said. “We are reinforcing areas to prevent their expansion and control of more villages and towns.”

The National Army compromises about 35,000 fighters from some of the biggest factions in a war that has killed hundreds of thousands of people and forced 11 million people from their homes over the last seven years.

The jihadists have been the dominant force in rebel-held Idlib province and also have a civil administration that runs basic services in many towns including Idlib city, the provincial capital.

Ideological differences divide them from nationalist groups in the Free Syrian Army gathered under the banner of the National Army and the National Liberation Front (NLF), both backed by Turkey.

The rebel cause to topple President Bashar al-Assad has been plagued by turf wars and local rivalries.

“Tahrir al Sham seek to end the presence of the NLF and control all of Idlib,” Hamoud said.

He echoed the view of military experts that the territorial gains would help widen the jihadists’ grip over a strip along the northern border with Turkey to their strongholds further south in the western Aleppo countryside.

Hamoud also accused the jihadists of attempting to divert their forces from any Turkish army campaign to seize the city of Manbij and Arab towns located east of the Euphrates River, controlled by the Syrian Kurdish YPG militias.

President Donald Trump’s abrupt decision to announce a U.S. pull-out from Syria has left open many questions, chiefly whether Kurdish fighters operating in northern Syria would now be targeted by their long-time enemy Turkey.