Turkish forces that entered Syria's Idlib province late on Thursday have started setting up observation positions that appear aimed at containing a Kurdish militia.
"The allied Turkish armed forces have entered Syrian territory and begun setting up observation posts," said Mustafa Sejari, an official in a Free Syrian Army (FSA) rebel group, on Friday.
Turkey sent a convoy of about 30 military vehicles into rebel-held northwest Syria through the Bab al-Hawa crossing in Idlib, according to rebel forces.
President Tayyep Erdogan announced the deployment last Saturday, saying Turkey was conducting a "serious operation" with rebel groups it supports, as part of a "de-escalation" deal it agreed last month with Iran and Russia in Kazakhstan.
Turkey has been a big supporter of rebels fighting Syrian president Bashar al-Assad throughout the war. But since last year Ankara has focused on securing its border against both extremists and from Kurdish forces that control much of the frontier area inside Syria.
The Astana agreement with Mr Assad's foreign allies Russia and Iran involves reducing warfare in several regions of Syria, including Idlib and adjacent swathes of the northwest, the most populous rebel-held area.
Tahrir al-Sham, a powerful jihadist alliance that controls much of that territory, is not party to the de-escalation deal, and is at odds with some of the rebel groups that Turkey supports.
However, its forces gave an escort to the Turkish convoy late on Thursday, indicating that there might not be any fighting between them.
Residents of northwest Syria and the war monitoring service, the Syrian Observatory for Human Rights, have reported intense air raids on Idlib and nearby areas for months.
Witnesses said the convoy was heading towards Sheikh Barakat, a high area overlooking rebel-held territory and the Kurdish YPG-controlled canton of Afrin, the witnesses said.
"(Turkey's deployment is) in line with Astana 6 resolutions to ensure the area is protected from Russian and regime bombing and to foil any attempt by the separatist YPG militias to illegally seize any territory," said Sejari.