New clashes erupted Friday between the Turkish army and a Syrian Kurdish militia seen as a terror group by Ankara but as a key ally by the United States in the fight against jihadists, the Turkish army said.
Rockets fired from an area in Syria controlled by the Kurdish Peoples' Protection Units (YPG) targeted a Turkish army command post in the Ceylanpinar district of Turkey's southern Sanliurfa province.
The Turkish army fired back, killing 11 "terrorists", it said. There were no reports of casualties on the Turkish side.
This was the third 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 has said it is "deeply concerned" that the strikes were conducted "without proper coordination either with the United States or the broader global coalition" against the Islamic State group (IS).
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 in advance.
Three armoured vehicles with American flags on Friday patrolled the area between the two border towns of Darabasiyah and Ras El Ein where cross-border clashes took place Wednesday between the Turkish forces and YPG.
They were accompanied with YPG forces on vehicles armed with heavy machine gun, according to an AFP correspondent near the border.
After the patrolling, the American forces went to the village of Ghanamiyah, south of Darabasiyah, where they were supposed to stay for several days to prepare a report about the situation, according to a source in the YPG.
The US-led coalition planes were flying overhead.
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 IS in Syria.
The new clashes came as President Recep Tayyip Erdogan on Friday warned the YPG that Turkey would fire back against any assault and thwart the creation of any Kurdish state in northern Syria.
"Are we going to leave them unanswered? We are doing what is necessary. We will take this kind of measure as long as the threats continue," Erdogan told a conference in Istanbul.