Turkish warplanes struck positions of a U.S.-backed Kurdish militia in Syria's Afrin province on Saturday, a senior Turkish official said, in a move likely to drive a deeper wedge between Ankara and the United States.
The operation opens a new front in Syria's war and sees Ankara confronting Kurds allied to the United States at a time when Turkey's relations with Washington are reaching breaking point.
The bombing raids targeted the Syrian-Kurdish YPG militia, the Turkish official said, adding that a Turkey-backed rebel group in Syria, the Free Syrian Army, was also providing assistance to the Turkish military's operation in Afrin.
The YPG said a number of people had been wounded in the air strikes.
The attacks follow weeks of warnings against the YPG in Syria from President Tayyip Erdogan and his ministers. Turkey considers the YPG to be an extension of the outlawed Kurdistan Workers Party (PKK), which has carried out a deadly, three-decade insurgency in Turkey's mainly Kurdish southeast.
The YPG's growing strength across a swath of northern Syria has alarmed Ankara, which fears the creation of an independent Kurdish state on its southern border.
In a statement, the Turkish military confirmed it had started an operation in Afrin, saying this was to provide safety for Turkey's border and to "eliminate terrorists... and save friends and brothers, the people of the region, from their cruelty."
Earlier on Saturday, the military said it hit shelters and hideouts used by the YPG and other Kurdish fighters, saying Kurdish militants had fired on Turkish positions inside Turkey.
But the U.S.-backed Syrian Democratic Forces - of which the YPG is a major component - accused Turkey on Saturday of using cross-border shelling as a false pretext to launch an offensive in Syria.
Differences over Syria policy have further complicated Turkey's already difficult relationship with NATO ally the United States. Washington has backed the YPG, seeing it as an effective partner in the fight against Islamic State.
A U.S. State Department official on Friday said military intervention by Turkey in Syria would undermine regional stability and would not help protect Turkey's border security.
Instead, the United States has called on Turkey to focus on the fight against Islamic State. Ankara accuses Washington of using one terrorist group to fight another in Syria.
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