ANKARA/BEIRUT (Reuters) – Syrian rebel fighters are launching a major military operation, backed by Turkish forces from inside Turkey, in a northern Syrian province largely controlled by jihadist militants, Turkish President Tayyip Erdogan said on Saturday.

The rebels said they were preparing to start the operation in Idlib soon, and residents reported Turkish authorities removing sections of a border wall.

The operation, part of a deal between Turkey, Iran and Russia to reduce warfare between rebels and the government, appears aimed at crushing the Tahrir al-Sham alliance, which has taken over much of Idlib province and northwestern Syria.

The three countries have supported opposing sides in Syria’s six-year conflict, with Turkey backing rebels fighting to overthrow President Bashar al-Assad, while Russian and Iranian military support helped Assad drive them back.

Erdogan’s comments, however, suggested Russia and Turkey would fight together against Tahrir al-Sham, an alliance led by the former al Qaeda affiliate in Syria that changed its name last year from the Nusra Front.

“There’s a serious operation in Syria’s Idlib today and it will continue,” Erdogan said in a speech to his AK Party, adding that Turkey would not allow a “terror corridor” on its border with Syria.

“For now Free Syria Army is carrying out the operation there,” Erdogan said. “Russia will be protecting outside the borders (of the Idlib region) and we will handle inside,” he said.

“Russia is supporting the operation from the air, and our armed forces from inside Turkey’s borders,” he added.

Mustafa Sejari, a senior official in the Liwa al-Mutasem Syrian insurgent group taking part in the operation, said Russian warplanes would not be militarily backing the rebels.

“As for the Russians, they will not have a role in the areas of our control at all. The role of the Russians is limited to areas under regime control,” he said.

Ankara, Moscow and Tehran announced a deal last month to establish and patrol a “de-escalation” zone in the Idlib region, where Erdogan has said Turkey will deploy troops, but Tahrir al-Sham pledged to keep on fighting.

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Turkey already has troops stationed inside Syria after it launched an incursion east of Idlib last year, known as Euphrates Shield, to drive back Islamic State militants and prevent further gains by Kurdish fighters on the border.

Syrian rebel officials from factions which have fought alongside Turkey in Euphrates Shield said they were preparing to enter the area with the backing of Turkish forces.

“The Free Syrian Army with support from Turkish troops is in full readiness to enter the area but until this moment there is no movement,” said Sejari, the Liwa al-Mutasem official.

Tahrir al-Sham is well entrenched in the border area in Idlib and maintains a big military presence in nearby towns, a local rebel said. The jihadist group has not yet commented on the Turkey-backed operation on its usual social media channels.

Another FSA rebel told Reuters he believed an incursion into northwest Syria was imminent. The Hamza Brigade, also part of Euphrates Shield, posted a video online of what it said was a convoy of its forces heading for Idlib.

Residents near the Bab al-Hawa border crossing with Turkey in Syria sent Reuters photographs of what they said was a section of the frontier wall being removed by the Turkish authorities.

Idlib’s population has ballooned to at least two million as thousands of civilians and combatants have left areas seized by the Syrian army in other parts of the country, with the help of Russian jets and Iran-backed militias.

Asked how far Turkey might go in deploying troops inside Syria, Erdogan declined to give details. “When you enter a boxing match, you don’t count how many punches you throw,” he said.