UPDATE 4-Turkey backs Syrian rebels for "serious operation" in Idlib
* Erdogan says Turkish troops will back op from inside Turkey
* Says will not allow "terrorist corridor" on frontier
* Rebels say campaign to start soon
* Tahrir al-Sham jihadists control much of Idlib (Adds quotes, background)
ANKARA/BEIRUT, Oct 7 (Reuters) - Syrian rebel fighters are launching a major military operation backed by Turkish forces into a northwestern area of Syria largely controlled by jihadists, Turkish President Tayyip Erdogan said on Saturday.
Rebels said they were preparing to start the operation soon in Idlib province and surrounding areas, which represent Syria's most populous rebel-held area, and residents reported Turkish authorities removing sections of a border wall.
The incursion is being carried out after Iran and Russia, which back President Bashar al-Assad, and Turkey, which supports the rebels, agreed last month to reduce fighting between insurgents and the government in the northwest.
It seems aimed at curbing the Tahrir al-Sham alliance, Syria's strongest jihadist group apart from Islamic State, and at securing Turkey's frontier.
"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 the Free Syrian Army is carrying out the operation there," Erdogan 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 Russia would not be backing the rebels in the campaign.
"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.
Tahrir al-Sham is spearheaded by the former Nusra Front, which was al Qaeda's Syrian branch until last year, when it changed its name and broke formal allegiance to the global movement founded by Osama bin Laden.
It has been a formidable military force since early in the war, when it transitioned from bomb attacks in cities to guerrilla conflict with the Syrian army, fighting alongside many other factions.
Since early this year, it has battled other rebel groups in Idlib and other parts of Syria, while the government has mostly focused on the war against Islamic State in the east.
In July Tahrir al-Sham seized land around the Bab al-Hawa border crossing between Syria and Turkey, bringing much of the frontier area inside Syrian under its control.
Tahrir al-Sham is well entrenched in the border area in Idlib and maintains a big military presence in nearby towns, a local rebel and a resident of a town near Bab al-Hawa said.
The jihadist group has not yet commented on the Turkey-backed operation on its usual social media channels.
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.
Tahrir al-Sham was also involved in fighting on the southern edge of the rebels' northwestern stronghold this week, battling the army north of Hama.
Air raids on insurgent-held towns in that area in recent days were intense, the Syrian Observatory for Human Rights, a Britain-based war monitor, reported.
Turkey has been one of the biggest supporters of rebels fighting Assad during the six-and-a-half year war, but its focus has evolved from ousting him to securing its border.
A year ago it backed Syrian rebels east of Idlib in an incursion known as Euphrates Shield to drive Islamic State and Kurdish groups from its border, sending troops and armor into Syria.
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.
Another FSA rebel told Reuters he believed an incursion into northwest Syria was imminent. "We expect that the large-scale operation that has been prepared will be launched perhaps within the next 24 hours," the rebel said.
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.
Idlib's population has ballooned to at least two million as thousands of civilians and combatants have left areas in other parts of the country seized by the Syrian army with the help of Russian jets and Iran-backed militias.
The resident of the town near Bab al-Hawa border crossing said people were worried that fighting would cause a big civilian toll because of the increased population there.
"One bullet kills three," the resident said.
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. (Additional reporting by Suleiman al-Khalidi in Amman, writing by Angus McDowall and Dominic Evans; editing by Clelia Oziel)
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