President Recep Tayyip Erdogan said on Saturday that Turkey and Syrian rebel groups it backs are starting a military operation in Syria’s Idlib province with the aim of pushing out militants from Hayat Tahrir al-Sham (HTS) that control the region.

Idlib and surrounding areas of northwest Syria are among the largest bastions for rebel groups fighting the regime of Bashar al-Assad, but have increasingly fallen under the sway of insurgent factions.

Erdogan said the operation, part of a de-escalation deal agreed between Turkey, Iran and Russia, will involve Free Syrian Army groups crossing into Idlib supported by Turkish soldiers from inside Turkey’s borders.

HTS is not party to the deal. 

“There’s a serious operation in Syria’s Idlib today and it will continue,” Erdogan said in a speech to his AK Party in the western city of Afyon.

“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 HTS alliance, spearheaded by formerly al-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.”

HTS has pledged to keep fighting Syrian regime 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 alliance.