ANKARA/BEIRUT (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. Late on Saturday, Turkey deployed tanks and military vehicles on the its Syrian border, building up military presence, a Reuters witness said. 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.  Turkey s foreign minister said late on Saturday that Turkey aimed to prevent clashes in Idlib.  Russian and Iranian observers will be in some regions here, and we will have our observers inside Idlib. Of course they will be in safe places so there will be no risks,  Mevlut Cavusoglu told reporters. 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. In a statement posted on social media, the jihadist alliance accused the Turkey-backed factions of working with Russia and described them as traitors, but did not mention Turkey.  It said in the statement that Idlib would  not be a picnic  for them and added  the lions of jihad and martyrdom are waiting to pounce .  Pentagon said the United States was not currently participating in these operations , but backed Turkey s efforts to fight terrorism, reminding that the U.S. stance on Tahrir al-Sham, remains unchanged.  We support our NATO ally Turkey s efforts to secure its borders, fight terrorism, and prevent safe havens for terrorist organizations,  Eric Pahon, a Pentagon spokesman, said. 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 take 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. 