Turkey’s military shelled Kurdish militia targets in northern Syria on Saturday, a Turkish government source and a monitor group said, hours after Prime Minister Ahmet Davutoglu warned that Ankara would act if it faced a threat from across the border. The shelling took place north of the city of Aleppo after 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, which Ankara considers a terrorist organization. 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. Turkey considers the PYD and YPG to be branches of the Kurdistan Workers’ Party (PKK), which has waged a decades-long insurgency against the country. A Kurdish official confirmed shelling had taken place at the Menagh air base, which he said had been captured by the Kurdish-allied Jaysh al-Thuwwar group rather than the YPG. Both are part of the U.S.-backed Syrian Democratic Forces alliance. The shelling took place amid growing anger in Ankara with U.S. backing for the PYD in its fight against Islamic State militants. The PYD is in control of most of the Syrian side of Turkey’s border, and nearby bases in Iraq’s Qandil Mountains have been bombed repeatedly by the Turkish military. Speaking Saturday in Erzincan, in eastern Turkey, Davutoglu said, “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.” Rising refugee numbers Turkey has been concerned by the tens of thousands of people fleeing to the Turkish border after attacks by Russian-backed Syrian government forces, increasing 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 stop its air support for Syrian government forces near 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 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. Foreign Affairs Minister Mevlut Cavusoglu, quoted in Turkish newspapers, said Riyadh and Ankara were coordinating plans to intervene in Syria, where Russia has been backing a successful regime offensive against rebels. “If there is a strategy [against the Islamic State jihadist group], then Turkey and Saudi Arabia could enter into a ground operation,” he said. Cavusoglu said Saudi Arabia was also sending planes to the Turkish base of Incirlik, a key hub for U.S.-led coalition operations against IS already used by Britain, France and the United States for cross-border air raids. World powers on Friday announced an ambitious plan to stop fighting in Syria within a week, but doubts have emerged about its viability, especially because it did not include the Islamic State militant group or al-Qaida’s local branch, which is fighting alongside other rebel groups in several areas.