The Turkish military said it began an operation late Thursday to set up "observation points" in Idlib, without providing further details or saying how many troops were involved.
"On October 12, we began activities to establish observation posts", the military said in a statement, after local media reported that Turkish troops and armoured vehicles entered Idlib province late Thursday.
Last week, Turkey announced an operation with Syrian opposition forces in Idlib to implement a de-escalation zone in the province. Brief clashes broke out along the border ahead of the deployment, amid reports of divisions within the insurgent coalition.
It was heading to Sheikh Barakat, a hilltop that overlooks large areas of rebel-held northwestern Syria, but also the Afrin area held by the Kurdish YPG militia.
The Turkish military operation in Idlib will also include Syrian rebel groups involved in the Euphrates Shield operation that Ankara launched in Syria previous year further to the east, Turkish President Tayyip Erdogan said on Saturday.
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While Turkey supports Syrian opposition fighters and calls for the removal of Syrian President Bashar al-Assad, Ankara has worked closely in the last few months with Russian Federation - who supports Assad - in the hope of bringing the war to an end. But since past year Ankara has focused on securing its border, both from jihadists and from Kurdish forces that control much of the frontier area inside Syria.
Turkey regards the YPG as an extension of the PKK, a Kurdish group inside Turkey that has been waging armed insurgency against Ankara for three decades.
The Britain-based Syrian Observatory for Human Rights said the deployment of almost 16 armored vehicles, five tanks and a military force came days after Turkey sent reconnaissance missions to the area.
Backed by the United States in its battle against Islamic State, the YPG has seized much of northeastern Syria and was trying to link that territory up with its canton in Afrin.
The Astana agreement with Assad's foreign allies Russian Federation and Iran involves reducing fighting in several regions of Syria, including Idlib and adjacent swathes of the northwest, the most populous rebel-held area.