Clashes in north-east Syria between Ankara-backed fighters supported by Turkish air power and the Kurdish-led Syrian Democratic Forces left 15 people dead on Saturday, a monitor said.
Rami Abdel Rahman, head of the UK-based Syrian Observatory for Human Rights, said nine pro-Turkish fighters and six SDF members were killed in a zone between the towns of Tel Tamer and Ras Al Ain.
Syrian state news agency Sana said government forces crossed the provincial border of Ras Al Ain near the frontier with Turkey on Saturday, an area that was taken by Turkish forces in a weeks-long offensive against Syria's Kurds.
The Observatory said the Syrian government's deployment there was its largest in years.
Syrian government troops also deployed along a road stretching about 30 kilometres south of the border, Sana said.
But Ankara's Defence Ministry said one Turkish military personnel was killed and five were wounded by a rocket and mortar attack carried out by the Kurdish People's Protection Units, or YPG.
The Turkish military was conducting reconnaissance work and responded to the attack, the ministry said.
Turkey and its Syrian proxies on October 9 launched a cross-border attack against Kurdish-held areas, grabbing a 120km stretch of Syrian land along the frontier.
The incursion left hundreds dead and caused 300,000 people to flee their homes, in the latest humanitarian crisis in Syria's eight-year civil war.
Turkey and Russia this week struck a deal in Sochi for more Kurdish forces to withdraw from the frontier on both sides of the Turkish-held area under the supervision of Russian and Syrian forces.
On Saturday, the Observatory said about 2,000 Syrian troops and hundreds of military vehicles were moving into what Turkey calls its "safe zone".
Government forces were being accompanied by Russian military police, the Observatory said.
Moscow has said that 300 Russian military police arrived in Syria to help ensure Kurdish forces withdraw to a line 30km from the border, in keeping with Tuesday's agreement.
Under the Sochi deal, Kurdish forces have until late Tuesday to withdraw from border areas at either end of the Turkish-held area, before joint Turkish-Russian patrols start in a 10km strip there.
Turkey eventually wants to set up a buffer zone on Syrian soil along the length of its 440km border with Syria. It also wants to resettle some of the 3.6 million Syrian refugees currently in Turkey.
Turkish President Recep Tayyip Erdogan gave a warning on Saturday that Ankara would "clear terrorists" on its border if the Kurdish forces, which his country view as an offshoot of its own PKK insurgency, did not withdraw by the deadline.
