New clashes erupted on Thursday between the Turkish army and a Syrian Kurdish militia regarded as a terror group by Ankara but seen by the United States as a key ally in the fight against extremists, reports said.
Mortar shells fired from an area in Syria controlled by the Kurdish Peoples' Protection Units (YPG) hit an army command post in the Akcakale district of Turkey's southern Sanliurfa province, the Dogan and Anadolu news agencies said.
The Turkish army fired back, killing three Kurdish fighters, it said. There were no reports of casualties on the Turkish side.
This was the second day in a row clashes have been reported across the tense border after the Turkish air force earlier this week bombed YPG targets in Syria.
The US State Department said it was "deeply concerned" that the strikes were conducted "without proper coordination either with the United States or the broader global coalition" against ISIS.
Russia's foreign ministry on Wednesday meanwhile said Turkey's bombing raids were unacceptable and called on all sides to show restraint.
But Ankara insisted that Washington and Moscow had been properly informed ahead of time.
Turkey says fighters of the YPG in Syria are linked to Kurdistan Workers' Party (PKK) separatists inside Turkey, who have waged an insurgency since 1984 that has killed over 40,000 people.
But Washington, wary of committing large numbers of its own forces on the ground, sees the YPG as essential in the fight against ISIS in Syria
