Clashes erupted between the Turkey-backed Free Syrian Army (FSA) and Syrian regime forces on Thursday, sources on the ground said.
The clashes were the first reported between the parties since the US-backed Syrian Democratic Forces (SDF) handed over villages west of the town of Manbij to the regime after Turkey threatened to attack if they stay in the area.
The SDF is an umbrella organisation of armed groups dominated by the YPG, which Turkey regards as an extension of the PKK, listed by Turkey, the US and the EU as a terrorist organisation.
The FSA said the fighting took place in two villages between al-Bab and Manbij, after regime forces and their allied militants attacked opposition positions.
Local sources told Reuters that the FSA captured two regime soldiers. Ten FSA soldiers were also injured in the clashes.
Regime-run SANA news agency reported that Turkey's military shelled its forces in the countryside near the northern city of Manbij, near the Turkish border.
The shelling caused deaths and injuries, regime sources said.
Turkey launched Operation Euphrates Shield in northern Syria in August 2016 in support of rebel groups with the twin goals of driving Daesh from its border and preventing the YPG from building an autonomous zone along the frontier.
Turkey has long demanded that the YPG move out of the Syrian town of Manbij, having previously warned the group not to advance west of the Euphrates river.
Turkish Armed Forces said on Friday it killed at least 71 YPG militants in northern Syria over the past week.