Turkish forces have entered northwest Syria's largely jihadist-controlled Idlib province, observers said, where Ankara said this week it planned to create a "de-escalation zone".
"A military convoy of Turkish forces... entered Idlib province before heading towards the western part of Aleppo province," the Britain-based Syrian Observatory for Human Rights said in a statement late Thursday.
It did not elaborate on the size of the deployment.
Turkish officials were not immediately available to confirm the report but the official Anadolu news agency earlier reported several armoured personnel carriers, ambulances and containers massing on the Syrian border.
The Turkish military said on Monday it had launched a reconnaissance mission into the area at the weekend "to establish surveillance posts as part of the operation to be carried out in Idlib province".
Turkish President Recep Tayyip Erdogan had on Saturday announced that pro-Ankara rebels backed by the Turkish army would lead the military campaign against the Hayat Tahrir al-Sham jihadist coalition in the northwestern Syrian province.
The operation is part of efforts by Turkey, along with Russia and Iran, to set up a de-escalation zone in line with accords in the Astana peace process aimed at ending the Syrian civil war.
They agreed on four such ceasefire zones in Syria as a prelude to negotiations.
Three zones are already in place -- in Eastern Ghouta near Damascus, in central Homs, and in parts of southern Syria -- and are being monitored by Russian military police.
Idlib is largely controlled by Hayat Tahrir al-Sham, a group led by Al-Qaeda's former Syria affiliate, which ousted more moderate rebels in recent months.
The pro-Turkish forces will need to oust Hayat Tahrir al-Sham fighters in the area to allow Iranian, Russian and Turkish forces to implement the zone.
The last time Turkish forces were engaged in Syria was in August, when Turkey launched its eight-month-long Euphrates Shield operation against jihadists and Kurdish militia in the northern Syrian province of Aleppo.
Although Moscow supports Syrian President Bashar al-Assad and Turkey supports rebels seeking his ouster, the two countries have worked together intensely to try to end the conflict in the past few months.
Erdogan and his Russian counterpart Vladimir Putin have met several times since a 2016 reconciliation ended a crisis caused by the downing of a Russian warplane over Syria.
The conflict in Syria began with widespread protests against the government in 2011 but has since evolved into a multi-front war that has killed more than 330,000 people.