Turkey's military shelled Kurdish militia targets in northern Syria on Saturday, a Turkish government source said, hours after Prime Minister Ahmet Davutoglu warned that Ankara would act if it faced a threat from across the border.
It was not clear why it launched the shelling, which took place after the Kurdish fighters, backed by Russian bombing raids, drove Syrian rebels from a former military air base near the Turkish border.
"The Turkish Armed Forces fired shells at PYD positions in the Azaz area," the source said, referring to the Kurdish Democratic Union Party (PYD), regarded by Ankara as a terrorist organisation.
The Syrian Observatory for Human Rights, which monitors Syria's war, said the shelling had targeted the air base and a village captured from insurgents by the PYD-backed YPG militia.
A Kurdish official confirmed the shelling of the Menagh base, which he said 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 US backing for the PYD in its fight against the Islamic State group militants.
The PYD 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 and whose bases in Iraq's Qandil mountains have been bombed repeatedly by the Turkish military.
"When there is any threat to Turkey, we will take in Syria the measures that we took in Iraq and in Qandil and will not hesitate to implement the necessary measures," Davutoglu said in a speech in the eastern Turkish city of Erzincan.
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 1,00,000.
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 condemned the attacks in Aleppo as "barbarity, tyranny, a war strategy conducted with a medieval mentality" and said hundreds of thousands of people in the region faced the danger of starvation if a humanitarian corridor is 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 Syrian President Bashar al-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 the Islamic State group.