Turkish forces have shelled Kurdish-held areas, including an air base, in Syria's northern province of Aleppo, sources have told Al Jazeera.
Syrian Kurdish fighters from the People's Protection Units (YPG) reported on Saturday that Turkish artillery targeted their positions in Menagh airport and a village near Azaz, which were recently captured from the Syrian opposition.
Turkish Prime Minister Ahmet Davutoglu confirmed on Saturday that Turkish forces had struck Kurdish YPG targets in northern Syria and demanded that the group withdraw from the area it recently captured.
"We will retaliate against every step [by the YPG]," he told reporters in comments shown live by state broadcaster TRT Haber. "The YPG will immediately withdraw from Azaz and the surrounding area and will not go close to it again."
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Al Jazeera's Zeina Khodr, reporting from the Turkish city of Gaziantep, near the Syria border, said the timing of the attack is "interesting", as it comes at a period when the YPG and their allies have been capturing rebel-held territory, taking advantage of an ongoing government offensive in Aleppo.
"Now, we can only speculate - was this a warning from the Turkish authorities who do not want to see the YPG expand control in Syria?" Khodr said.
"In the past, Turkey has fired shells across its border but usually in retaliation - after a shell lands on Turkish soil, or if fighting is getting closer to its border they fire shells as a deterrent."
US State Department spokesman John Kirby urged Turkey on Saturday to halt military strikes on Kurdish and Syrian regime targets in the northern province of Aleppo.
"We are concerned about the situation north of Aleppo and are working to de-escalate tensions on all sides," Kirby said in a statement.
The shelling came after Davutoglu said on Saturday that Ankara would, if necessary, take military action against fighters from the Syrian Kurdish Democratic Union Party (PYD).
"We can if necessary take the same measures in Syria as we took in Iraq and Qandil," Davutoglu said in a televised speech in the eastern city of Erzincan, referring to the bombing campaign last year against Kurdistan Workers Party (PKK) targets in northern Iraq on their Qandil mountain stronghold.
"We would expect our friends and allies to stand by us," he added.
Turkey considers the PYD and its YPG militia to be branches of the PKK, which has waged a decades-long armed campaign against the Turkish state.
Earlier this week, Turkey reacted furiously to comments by the US State Department spokesman saying Washington did not recognise the PYD as a "terrorist" group and would continue to support its operations in Syria.
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"The leadership cadre and ideology of the PKK and PYD is the same," Davutoglu said on Saturday.
"Those who say that they are not terror groups either do not know the region or have bad intentions. We will be sending documentation to the United States very soon to show that the PYD is a branch of the PKK."
Washington recognises the PKK as a "terrorist" group, as does the European Union.
Last week, Kirby said that Kurdish fighters "have been some of the most successful" in fighting the Islamic State of Iraq and the Levant group inside Syria.
He said the U.S. had supported the Kurdish fighters, mostly with air power, "and that support will continue".
With reporting from Al Jazeera's Diana Al Rifai
Source: Al Jazeera and agencies