AMMAN (Reuters) - Blacklisted jihadist fighters tightened their grip over Syria’s last major rebel-held region on Thursday after nine days of battle with Turkey-backed groups, putting a deal to prevent a massive army assault on the area in jeopardy.

The northwest of Syria near the Turkish border is the last part of the country still in the hands of fighters seeking to topple President Bashar al-Assad, but control has been divided between jihadist factions and other rebels backed by Turkey.

On Thursday Hayat Tahrir al-Sham (HTS), a jihadist group listed as terrorists by the United States, Turkey and others, forced factions from the Turkey-backed Free Syrian Army (FSA) to accept a peace deal recognizing civilian control by an HTS-backed administration.

The success of the jihadists in recent days raises doubt over the future of a deal agreed in September between Turkey and the Assad government’s main ally Russia to avert an army assault. The agreement requires banned jihadist groups to be expelled from a frontline buffer zone.

Much of the Idlib enclave is now controlled by HTS, which is spearheaded by the former Syrian offshoot of al Qaeda.

Under Thursday’s agreement, FSA groups accepted civilian control of some towns and villages by a body known as the Salvation Government which runs basic services in the city of Idlib and many towns that fall under HTS influence.

Turkish Foreign Minister Mevlut Cavusoglu acknowledged on Thursday it was not “so easy” to maintain the deal with Russia, but said so far it was being “implemented successfully”.

“The radical groups have mounted an attack against the moderate opposition. And of course we are taking the necessary precautions,” Cavusoglu said in an interview with broadcaster NTV.

A rebel official close to Turkey’s intelligence service said Ankara had played a key role in preventing the fighting from spreading further by pressing rebels to accept a deal.

HTS’s stranglehold over the area is raising fears among rebels and residents in the heavily populated province that Russian air strikes halted last year could resume.

The jihadists have allowed Turkish troops to deploy along the front lines as the Russia-Turkey deal demanded, but have not pulled out of the area themselves.

“The complete control of (HTS) will be a pretext for the regime and the Russians to end the Idlib deal, and this is the coming danger,” said Major Youssef Hamoud, spokesman for a Turkey-backed alliance of rebels called the National Army.

Some factions who fought against the jihadists under a different banner have now fled to an area under greater Turkish influence nearby.