Turkish President Recep Tayyip Erdogan on Saturday announced pro-Ankara Syrian rebels were launching a new military operation in Syria's northwestern Idlib province to push out the jihadists who control the region.
The move comes as Turkey, along with Russia and Iran, prepare to set up a so-called "de-escalation" zone in Idlib in line with accords in peace talks in Astana aimed at ending the Syrian civil war.
Turkish commando units and military vehicles have reportedly massed on the border but are yet to cross, in what will be Ankara's second major Syria operation after its Euphrates Shield incursion last year.
"We are taking new steps to ensure security in Idlib. Today, a very serious operation is ongoing in Idlib and this will continue," Erdogan said in a televised speech in the western city of Afyon.
He later told reporters the operation was led by Free Syrian Army (FSA) rebels and that the Turkish army was "not yet" operating inside Syria.
Aron Lund, fellow with The Century Foundation think tank, told AFP any Turkish operation "is likely to rely on a mix of Turkish and Syrian forces".
He said Turkey would contribute special forces, logistics, artillery and tanks but Syria rebels would make up "much of" the ground force.
Idlib is largely controlled by Hayat Tahrir al-Sham (HTS), a group led by Al-Qaeda's former Syria affiliate, which ousted more moderate rebels in recent months.
HTS is not party to a deal brokered by Russia, Turkey and Iran for the safe zone in the province, one of four such "de-escalation" zones across Syria.
"If Turkey decides to go head to head with HTS, I'm sure they will meet severe resistance," Lund told AFP.
Ousting HTS forces from the area will be needed to allow the arrival of Iranian, Russian and Turkish forces to implement a de-escalation zone.
"We will absolutely not allow the creation of a terror corridor along our borders," said Erdogan.
State-run Anadolu news agency said there was a major build up of commando units and military vehicles around the town of Reyhanli bordering Idlib close to the Cilvegozu border crossing.
The Hurriyet daily said ultimately Turkey would ensure security in Idlib city and Russia in the surrounding area.
Appearing to confirm this, Erdogan said: "Idlib is a region where we will provide protection in the inside and Russia on the outside."
Turkish army cranes had begun removing sections of the security wall Turkey has built on the border in apparent preparation for an incursion, an AFP photographer said.
Turkey earlier this year wrapped up its half year Euphrates Shield operation against jihadists and Kurdish militia in Aleppo province that involved both the Turkish army and Syrian rebels.
"Despite all the provocations and the obstacles, we have made significant progress in Syria," said Erdogan.
A rebel commander participating in the operation, who spoke on condition of anonymity, told AFP in Beirut that "all the rebel groups" who took part in Euphrates Shield are participating.
"The goal of the operation is to liberate Idlib fully from Tahrir al-Sham," he said.
Turkey has also long warned it could also move against Kurdish militia in Afrin to the east. Erdogan warned that "new initiatives" would follow after the Idlib operation.
But Ankara is playing no part in the US-backed assault to prise the Syrian city of Raqa from the Islamic State group due to the involvement of Kurdish militia Turkey considers a terror outfit.
The Idlib operation comes a week after Russian President Vladimir Putin held talks with Erdogan in Ankara, with both sides agreeing to push for the Idlib de-escalation zone.
Afer those talks Putin declared the right conditions now existed to end the over six-year civil war that has killed an estimated 330,000 people since 2011.
Despite being on opposite sides of the conflict, Russia and Turkey have been working together intensely since a 2016 reconciliation deal ended a crisis caused by the shooting down of a Russian war plane over Syria.
Russia, along with Iran, is the key backer of President Bashar al-Assad and Moscow's military intervention inside Syria is widely seen as tipping the balance in the conflict. Turkey, however, has backed rebels seeking Assad's ouster.
Commenting on the coordination with Russia, Erdogan said: "Relations with the regime is something looked after by Russia, and we have taken measures in other areas."