Turkish soldiers and armoured vehicles have crossed into Syria as part of an operation Ankara says is aimed at stopping fighting in Idlib province.
A military statement said the forces would set up observation posts to help monitor a "de-escalation zone" in line with an agreement with Russia and Iran.
Idlib is largely controlled by Hayat Tahrir al-Sham (HTS), a jihadist alliance linked to al-Qaeda.
It also borders the Kurdish enclave of Afrin, which Turkey wants to contain.
Turkey backs the opposition to Syrian President Bashar al-Assad and last year supported a rebel offensive against so-called Islamic State (IS) along the border in neighbouring Aleppo province.
That operation also served to prevent a US-backed alliance that is led by the Kurdish Popular Protection Units (YPG) seizing the territory and reaching Afrin.
Ankara considers the YPG an extension of the banned Kurdistan Workers' Party (PKK), which has fought for Kurdish autonomy in Turkey for three decades.
Turkey's state-run Anadolu news agency reported that military convoys had crossed into Idlib from Reyhanli in the southern Turkish province of Hatay.
The Hurriyet newspaper said about 30 armoured vehicles and more than 100 soldiers were taking part in the operation, and speculated that more troops could be sent in the coming days.
The Turkish military statement said its soldiers had started their activities in Idlib by establishing observation posts in line with a plan agreed last month with Russia and Iran - which back President Assad - in the Kazakh capital Astana.
The three powers said they would each send about 500 observers to positions along the edge of the de-escalation zone to prevent clashes between Syrian government and rebel forces, allow the delivery of humanitarian aid and establish conditions necessary for the return of displaced people.
HTS, which was formed in January by a group previously known as al-Nusra Front, warned that the move would "end with restoring Bashar's rule" to Idlib.
However, the UK-based Syrian Observatory for Human Rights reported that HTS fighters had escorted the Turkish troops towards a western area adjoining Afrin.
A Kurdish militiaman told the Associated Press that the troops were stationed in three separate points along a "front" between Afrin and Idlib.
An official in a Free Syrian Army (FSA) faction said the Turkish deployment would "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".
Turkish President Recep Tayyip Erdogan vowed last week that he would not allow the YPG to form a "terror corridor" to the Mediterranean, which would require the Kurds capturing mountainous areas held by opposition and government forces.
CNN Turk reported that there had been a clash in the Idlib countryside near the Ogulpinar border crossing, but it was not clear which forces were involved.