Several Turkish military vehicles, ambulances and tankers were visible in photographs published by Turkey's Anadolu news agency late on Thursday stationed at a village near Turkey's Reyhanli border gate opposite Bab al-Hawa.
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.
"On October 12, we began activities to establish observation posts", the military said in a statement. "Tonight, Turkish Armed Forces jointly with the FSA units started an operation in Idlib", the president said.
The army said its forces in Syria were conducting operations in line with rules of engagement agreed with Russian Federation and Iran.
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However, the Turkish deployment is also meant to rein in the Kurdish YPG militia, which holds the Afrin region next to Idlib, a senior rebel official involved in the operation said.
The fourth de-escalation zone includes Idlib but also parts of the neighboring Latakia, Hama and Aleppo regions.
The operation's target is the HTS but Observatory head Rami Abdel Rahman told AFP in Beirut that Turkish forces were escorted by HTS elements, appearing to suggest a degree of coordination by Turkey with the group.
In recent weeks, a military build-up had taken place on Turkey's border with Syria, with Turkeysupporting a campaign to secure opposition control over Idlib.
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A total of 14 "observation posts" will be set up by the Turkish army, according to local media, while some 500 Turkish soldiers could be deployed to Idlib.
A large column of Turkish tanks and armoured vehicles has crossed into northern Syria in an operation described as a peacekeeping effort by Ankara but criticised by the Assad regime as a land grab.
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.
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