Three Turkish soldiers have been killed by gunfire in the town of Ceylanpinar on the border with Syria, Turkish news agencies have reported.
It was unclear who the attackers were and whether the firing came from across the border, the agencies say.
Ceylanpinar is close to the Syrian town of Ras al-Ain. The two countries are separated by a barbed wire fence.
Security has been tightened following the shooting. The region is mostly Kurdish on both sides of the border.
But Islamic State (IS) militants also control much of the Syrian border area adjoining Turkey.
"We are investigating whether the gunfire came from the Syrian side or not. Our extensive investigation in the area is ongoing. We will have a clearer explanation [later] when the day lightens up," the Hurriyet website quoted local governor Izzettin Kucuk as saying.
Clashes between the army and the Kurdish separatist PKK have taken place regularly in the region in recent years.
However, in October Turkey allowed some 150 Kurdish Peshmerga forces to cross its border and help defend the Syrian town of Kobane from IS forces.
Turkey regards the PKK as a terrorist organisation and its leader Abdullah Ocalan is in jail. But he has been Turkey's main point of contact in peace negotiations since he was imprisoned in 2012.
The PKK - also labelled as terrorist by some Western governments - has been waging a 30-year insurgency for self-rule in eastern Turkey. The unrest has killed more than 40,000 people.