Turkey said on Saturday that it and Syrian rebel groups it backs are starting a military operation in Syria's Idlib, where a jihadist alliance is blocking efforts to impose a truce between insurgents and the Syrian government.
Idlib and surrounding areas of northwest Syria are among the largest bastions for rebel groups fighting Syrian President Bashar al-Assad, but have increasingly fallen under the sway of jihadist insurgent factions.
Turkish President Tayyip Erdogan said the operation, part of a de-escalation deal agreed between Turkey, Iran and Russia, will involve Syrian rebel groups crossing into Idlib supported by Turkish soldiers from inside Turkey's borders.
"There's a serious operation in Syria's Idlib today and it will continue," Erdogan said in a speech to his AK Party.
"Now this step has been taken, and it is underway," he said, adding that Turkish forces were not yet involved and that it was a rebel operation so far.
Russia, an ally of Assad, is backing the operation from the air, he said.
Much of Idlib is controlled by the jihadist Tahrir al-Sham alliance, spearheaded by a former al Qaeda affiliate that changed its name last year from the Nusra Front.
"We will never allow a terror corridor along our borders in Syria," Erdogan said. "We will continue to take other initiatives after the Idlib operation."
Tahrir al-Sham has pledged to keep fighting Syrian government forces and their allies, casting doubt on the de-escalation agreement, but Ankara has worked to lure militants from it and two groups have defected from the jihadist alliance.
Turkey launched an incursion into Syria further east of Idlib last year, backing Free Syrian Army (FSA) groups in the Euphrates Shield operation against Islamic State and Kurdish groups.
The FSA groups supported by Turkey are now ready to enter Idlib, Mustafa Sejari, a senior official in the Liwa al-Mutasem group said.
"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," he said.
Erdogan said last month that Turkey would deploy troops in Idlib province as part of the de-escalation agreement brokered by Russia in August.
Another FSA rebel in the Euphrates Shield campaign told Reuters he believed an incursion into northwest Syria was imminent.
The Hamza Brigade, also part of Euphrates Shield, posted 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.