Istanbul - The Turkish army struck positions of Kurdish fighters inside Syria for a second day Sunday in response to incoming fire, state media said, as an explosive stand-off between Ankara and
The army hit Democratic Union Party (PYD) targets around the Syrian town of Azaz using howitzers stationed on the Turkish side of the border, the state-run Anatolia news agency reported.
Turkish forces carried out a similar bombardment on Saturday.
Anatolia gave no further details but military sources quoted by Turkish media said the new exchange took place from 07:00 GMT.
Turkish forces would continue to strike PYD targets in Syria as long as the army came under fire from their positions, the military sources added.
Turkish media reports said the fire was clearly audible from the border crossing close to the southern Turkish town of Kilis.
NTV television said Turkish forces had also captured three suspected PYD members in Kilis, without giving further details.
In Damascus, the Syrian government that is strongly opposed by Turkey condemned the attacks on its "territorial integrity" and urged the United Nations to act, Syrian state media said.
'Won't watch from sidelines'
Turkey has been gravely alarmed by the push by the PYD and its People's Protection Units (YPG) militia westwards along the Syrian border to the flashpoint town of Azaz.
Ankara accuses the PYD of being the Syrian branch of the Kurdistan Workers' Party (PKK) which has waged a decades-long insurgency against the Turkish state that intensified in the last few months.
But Washington has been relying on the YPG as one of its few effective allies on the ground in the fight against Islamic State (ISIS) jihadists in northern Syria.
The PKK is recognised by the United States as a terror group but not the YPG or PYD.
The issue is causing increasing friction between the two NATO allies and complicating efforts to find a solution to Syria's five year civil war.
US State Department spokesperson John Kirby said in a statement late Saturday that Washington urged Turkey to cease its cross-border artillery fire.
He said the United States had also urged the Kurdish fighters "not to take advantage of a confused situation by seizing new territory".
Turkish Deputy Prime Minister Yalcin Akdogan said that the movement of the YPG west of the Euphrates river in Syria was a "red line" for Ankara to which Turkey would never give its assent.
"We are talking here about issues that concern Turkey's national security," he told Kanal 7 television. "Turkey is not a nation that is going to sit down and watch everything from the sidelines or the stands."
Prime Minister Ahmet Davutoglu said on Saturday that Turkey would take any step necessary against the PYD, IS jihadists or the Syrian regime.
"We will take every necessary step to ensure an environment that guarantees Turkey's security, an environment without Daesh (ISIS), the Syrian regime or the PYD," he said in the eastern city of Erzincan, quoted by Anatolia.
With Turkey appearing to ramp up its involvement in the Syrian crisis, Foreign Minster Mevlut Cavusoglu said Saturday Ankara and its ally Saudi Arabia could launch a ground operation against IS.
Saudi Arabia also has deployed aircraft to Turkey's Incirlik airbase to "intensify" its operations against ISIS, a senior Saudi defence official said.