Turkish troops on Sunday exchanged fire with Syria-based jihadists as Ankara massed military vehicles on the frontier ahead of an expected operation to oust al-Qaeda's former Syrian affiliate from Idlib province.
President Recep Tayyip Erdogan on Saturday announced the launch of an operation by pro-Ankara Syrian rebel forces, backed by the Turkish army, to reimpose security in Idlib.
Most of the northwestern region is 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.
Turkey has massed special forces and military hardware including tanks on the border but the operation has yet to begin in earnest, monitors and sources on the ground said.
But Turkish forces fired seven mortars over the border with the aim of easing the passage of the pro-Ankara Syrian forces, the Dogan news agency reported.
Turkish forces have also been seen removing parts of the security wall Ankara has built on the border so that military vehicles can pass through into Syria.
Pro-government media said that the operation was now into its "second day" and it was not immediately clear what the Turkish military's next move would be.
On Sunday morning, HTS jihadists opened fire on Turkish forces removing part of a wall along the border between Turkey and Idlib, witnesses and the Syrian Observatory for Human Rights monitor said.
The Observatory reported "heavy exchanges of fire", but said the incident did not appear to mark the start of the operation Erdogan described on Saturday.
Television images showed locals in the Turkish border town of Reyhanli in Hatay province cheering as more armoured vehicles were driven through the town overnight.
Ankara appears keen to oust the HTS from Idlib in order to create a de-escalation zone into which it can send military monitors to implement a ceasefire.
Turkey, along with Syrian regime allies Russia and Iran, earlier this year agreed a deal to implement four such ceasefire zones in Syria as a prelude to talks on a peace deal.
The zone encompassing Idlib is the last one to go into effect, and its implementation has been held up by fierce opposition from HTS.
On Saturday, the group warned "treacherous factions that stand by the side of the Russian occupier" should only enter the area if they want "their mothers to be bereaved, their children to be orphaned, their wives to be widowed".