Turkey's military shelled Kurdish militia targets in northern Syria on Saturday and Prime Minister Ahmet Davutoglu demanded that the group withdraw from the area in a move that further complicated the conflict across the NATO member's border.
The shelling took place after Kurdish YPG fighters backed by Russian bombing raids drove Syrian rebels from a former military air base, south of the town of Azaz and near the Turkish border.
"Today retaliation was taken under the rules of engagement against forces that represented a threat in Azaz and the surrounding area," the prime minister told reporters in comments shown live by state broadcaster TRT Haber.
A Kurdish official said the Menagh base that was hit had been captured by the Kurdish-allied Jaysh al-Thuwwar group rather than the YPG. Both are part of the Syria Democratic Forces alliance.
The shelling came amid growing anger in Ankara with the United States for supporting the YPG, which Ankara regards as a terrorist organization, in its fight against Islamic State militants.
The Kurdish Democratic Union Party (PYD), which backs the YPG, controls most of the Syrian side of Turkey's border and Ankara views it as an extension of the Kurdistan Workers Party (PKK), which has waged a three-decade-old insurgency for autonomy in southeast Turkey.
US State Department spokesperson John Kirby urged both Turkey and the Syrian Kurds to step back, saying they should focus instead on tackling a "common threat" of Islamic State militants who control large parts of Syria.
"We have urged Syrian Kurdish and other forces affiliated with the YPG not to take advantage of a confused situation by seizing new territory," Kirby said in a statement.
"We have also seen reports of artillery fire from the Turkish side of the border and urged Turkey to cease such fires."
Davutoglu demanded that the Menagh base be evacuated and said he had spoken to US Vice President Joe Biden to make that point and stress that the PYD was an extension of the PKK and a direct threat to Turkey.
"We will retaliate against every step (by the YPG)," he said after a visit to the eastern Turkish city of Erzincan. "The YPG will immediately withdraw from Azaz and the surrounding area and will not go close to it again."
Turkey's disquiet has been heightened by the tens of thousands of people fleeing to the Turkish border after attacks by Russian-backed Syrian government forces, swelling refugee numbers in the area to 100,000.
Turkey, which already hosts 2.6 million Syrian refugees, has kept the latest arrivals on the Syrian side of the border, in part to pressure Russia to cease its air support for Syrian government forces near the city of Aleppo.
Davutoglu earlier condemned the attacks in Aleppo as "barbarity, tyranny, a war strategy conducted with a medieval mentality" and said hundreds of thousands faced the danger of starvation if a humanitarian corridor was not opened.
"We will help our brothers in Aleppo with all means at our disposal. We will take those in need but we will never allow Aleppo to be emptied through an ethnic massacre," he said.
NATO-member Turkey is one of Assad's most vehement critics and an ardent supporter of opposition forces.
Turkish Foreign Minister Mevlut Cavusoglu was reported as saying on Saturday that Saudi Arabia would send aircraft to Turkey's Incirlik air base for the fight against Islamic State.
