efe-epaCeylanpinar, Turkey-Syria border

Casualties are mounting in Turkey's military offensive against Kurdish forces in northern Syria, according to reports from both sides of the conflict Friday.

NATO's secretary-general Jens Stoltenberg, who earlier met with Turkish Foreign Minister Mevlut Cavusolgu in Istanbul, has expressed his concerns that the offensive could lead to further instability in the region. Turkey is NATO's second largest member in terms of military power, after the United States.