Turkish artillery fired into Syria's Afrin region on Friday in what Ankara said was the start of a military campaign against the Kurdish-controlled area.
The cross-border bombardment took place after days of threats from Turkish President Recep Tayyip Erdogan to crush the Syrian Kurdish YPG, or People's Protection Unit, militia in Afrin in response to what Turkey sees as growing Kurdish strength across a wide stretch of north Syria.
"The operation has actually de facto started with cross-border shelling," Turkish Defence Minister Nurettin Canikli said.
Canikli said no troops had crossed into Afrin.
Direct military action against territory held by Kurdish militia would open a new front in Syria's civil war and would see Ankara confronting Kurds allied to the United States at a time when Turkey's relations with Washington are reaching the breaking point.
The U.S. State Department has called on Turkey to focus on the fight against ISIS militants and not take military action in Afrin. An official from the department said such moves would undermine regional stability and would not help protect Turkey's border security.
"We do not believe that a military operation serves the cause of regional stability, Syrian stability or indeed Turkish concerns about the security of their border," the official told reporters, stressing he had limited information about Turkey's reported military moves. "The kind of threats or activities which these initial reports may be referring to, we don't think advance any of these issues. They are destabilizing."