Turkish forces exchanged fire Sunday with jihadists from Al-Qaeda's former Syrian affiliate on the border of Idlib province, a monitor and eyewitnesses said, a day after Ankara announced an imminent operation there.

Turkish President Recep Tayyip Erdogan on Saturday announced pro-Ankara rebels would lead a military campaign against the Hayat Tahrir al-Sham jihadist coalition in the northwestern Syrian province.

On Sunday morning, HTS jihadists opened fire on Turkish forces removing part of a wall along the border between Turkey and Idlib, eyewitnesses and the Syrian Observatory for Human Rights monitor said.

"A group of HTS opened fire on the vehicle removing part of the wall, and the Turks returned fire and also shelled the area," one eyewitness on the border told AFP.

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.

Turkish shelling hit near a camp for displaced civilians by the border, prompting some to flee the area, it said, and HTS fighters also shelled a Turkish position near the Bab al-Hawa border crossing.

Turkey's NTV, a private television station, reported an exchange of fire at the border area, quoting "military sources".

On its website, it said the Turkish army had fired artillery in support of the allied Syrian rebels.

The campaign against HTS has been the subject of weeks of speculation, and is linked to plans to implement a so-called "de-escalation zone" in Idlib province and surrounding areas.

Rebel backer Turkey, along with Syrian regime allies Russia and Iran, earlier this year agreed a deal to implement four such ceasefire zones in the war-torn country 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, which is dominated by Al-Qaeda's former Syrian affiliate.

The group controls almost all of Idlib province after ousting rebel groups, including former allies, in an assault earlier this year.

On Saturday, it 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".

More than 330,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.