The Turkish army has claimed it has killed 11 "terrorists" during cross-border clashes with Syrian Kurdish fighters.

Rockets fired on Friday from an area controlled by the Kurdish Peoples' Protection Units (YPG) in Syria targeted a Turkish army command post in the Ceylanpinar district of Turkey's southern Sanliurfa province.

The Turkish army fired back and killed 11 "terrorists", it said. There were no reports of casualties on the Turkish side.

This was the third day in a row that clashes have been reported across the tense border after the Turkish military jets carried out air strikes targeting the YPG fighters in the area earlier this week.

Friday's clashes came as President Recep Tayyip Erdogan 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," Erdogan said in Istanbul.

"We will take this kind of measure as long as the threats continue."

Ankara sees YPG as "a terror group" linked to outlawed Kurdistan Workers' Party (PKK) inside Turkey. The PKK has waged a battle against Turkish authorities since 1984.

But the United States considers it to be a key ally in the fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) in Syria.

A Turkish military statement said Tuesday's strikes were carried out to prevent the PKK from sending "terrorists, arms, ammunition and explosives to Turkey", adding that the operation was conducted "within the scope of the international law".

The US Department of State has said it is "deeply concerned" Tuesday's strikes - that were also carried out in Iraq - were conducted "without proper coordination either with the United States or the broader global coalition" against ISIL.

On Wednesday, Russia's foreign ministry said the attacks carried out by Turkey were unacceptable and called on all sides to show restraint. Ankara said Washington and Moscow had been properly informed in advance.