Mortar shells fired from an area in Syria controlled by the YPG hit an army command post in the Akcakale district of Turkey's southern Sanliurfa province on Thursday, the second day of clashes across the tense border.
The Turkish army fired back, killing three YPG militants, the Dogan and Anadolu news agencies said. There were no reports of casualties on the Turkish side.
The YPG is the Syrian wing of the PKK, designated a terrorist organisation by Turkey, the US and EU. The PKK has been waging a deadly insurgency in Turkey since 1984 that has killed over 40,000 people.
The US, however, backs the YPG as a key component of the Syrian Democratic Forces (SDF) which have been closing in on the Daesh bastion of Raqqa.
The attack on the army post in Akcakale comes after Turkish war planes struck YPG positions in northeastern Syria on Tuesday in what is part of a widening campaign against terror groups affiliated with the PKK. YPG militants targeted Turkish military outposts on the border with Syria in southern Hatay and southeastern Mardin provinces a day later.