BEIRUT (Reuters) - A jihadist assault gained ground on Tuesday against Turkish-backed rebels in northwest Syria, edging closer to frontlines with government forces, a rebel official and a war monitor said.

The advance raises questions about the fate of September’s demilitarization deal between Turkey and Russia, which staved off a Syrian army offensive against the Idlib region.

Hayat Tahrir al Sham (HTS), spearheaded by al Qaeda’s former Syrian affiliate, is expanding its grip over the insurgent enclave, which includes Idlib province and adjacent bits of Aleppo, Hama and Latakia provinces.

Idlib lies in Syria’s northwest corner, the last opposition stronghold, where Turkish forces are stationed. It borders territory that Turkey-backed rebels control near the Turkish frontier.

The main Turkish-backed rebel force, the National Army, has deployed along fronts close to the jihadists to repel any new militant advance towards them.

National Army spokesman Major Youssef Hamoud said HTS militants seized four villages in al-Ghab plain from rival rebels on Tuesday.

“We call on (the rebel factions) inside Idlib to launch an operation so that we can try to open a new front and relieve some pressure off them,” he said.

He said HTS jihadsists, who launched their offensive last week, were preparing to march towards the key towns of Ariha and Maarat al-Numan in Idlib.

The Syrian Observatory for Human Rights said fierce battles raged in al-Ghab plain. The British-based monitoring group said if HTS did take Ariha, Maarat al-Numan and some villages in between, it would effectively control all Idlib.

The fighting was taking place inside a buffer zone that the Russian-Turkish deal had established along the front lines, the Observatory and a resident said.

The resident of Sahl al-Ghab said Turkish-backed factions had “no option but to fight to the death”.

“The factions are enclosed in a very small area.” HTS gains had displaced some civilians towards government territory, the resident added.