A Turkish military convoy entered rebel-held northwestern Syria early on Wednesday, an AFP correspondent reported, as the deadline for establishing a buffer zone between rebel and government forces draws closer.
At least 40 vehicles, including trucks and armoured personnel carriers, were seen moving slowly south along a main highway under the cover of darkness.
The troops they were carrying are expected to be deployed at "observation posts" Turkey has already set up in rebel-held areas of Idlib and Aleppo provinces.
The Syrian Observatory for Human Rights, a Britain-based war monitor, said the vehicles were heading to Turkish monitoring posts, including in the southwestern parts of Idlib.
There was no word from Turkey on any reinforcements to the area. It last sent a similar troop column into northern Syria just over a week ago.
Wednesday's convoy entered as the deadline for implementing a Turkish-Russian accord on the future of Syria's last major rebel bastion draws closer.
Last month, Moscow and Ankara agreed to set up a demilitarised zone ringing rebel territory to avert a threatened government offensive.
All factions in the zone must withdraw heavy weapons by October 10, and radical groups must leave by October 15. It will then be monitored by Turkish troops and Russian military police.
The region's dominant armed group, Hayat Tahrir al-Sham (HTS), an alliance led by jihadists of Syria's former Al-Qaeda affiliate, has still not officially responded to the accord.
Turkey's main rebel allies, the National Liberation Front, gave the deal a cautious welcome before objecting to its provisions for a Russian troop presence inside the zone.
The NLF says it is still hammering out the details of the agreement's implementation with Ankara, and is wary that the current proposals would eat into their territory too much.
In an interview aired Tuesday, Syrian Foreign Minister Walid Muallem said he hoped the deal would prove to be a "step towards the liberation of Idlib."
Muallem told Lebanon's Al-Mayadeen television he was confident in Turkey's ability to fulfil its side of the deal "because of its knowledge of factions" on the ground.
Fighters from the area would be allowed to stay, he said, while those from other areas would go home and foreigners would leave through Turkey.