The rapid advance of U.S.-backed Kurdish fighters in northern Syria, taking advantage of Russian air strikes to seize territory near the Turkish border, has infuriated Ankara and threatened to drive a wedge between NATO allies.
Washington has long seen the Syrian Kurdish Democratic Union Party (PYD) and its YPG military wing as its best chance in the battle against Islamic State in Syria - to the chagrin of fellow NATO member Turkey, which sees the group as terrorists and fears it will stir up greater unrest among its own Kurdish minority.
Russian bombing has transformed the five-year-old Syrian civil war in recent weeks, turning the momentum decisively in favour of Moscow's ally President Bashar al-Assad.
The Syrian army has come within 25 km (15 miles) of the Turkish border and says it aims to seal it off altogether, closing the main lifeline into rebel territory for years and recapturing Aleppo, Syria's largest city before the war.
Meanwhile, the YPG has exploited the situation, seizing ground from other Syrian opposition groups in the area.
Washington says it does not believe the Kurds are coordinating directly with Moscow. But the YPG's advance may represent a masterstroke by Russia, which could benefit from any discord between NATO allies Turkey and the United States.
"Now this is the YPG's dilemma: Will it continue with America or Russia? The consequences of this strategic choice will influence Syria's future, as well as the ongoing clashes in Turkey," said Metin Gurcan, an independent security analyst and retired Turkish military officer.
Turkey has shelled YPG positions inside Syria for four straight days. Ankara sees the militia as an extension of the outlawed Kurdistan Workers Party (PKK), which has fought a three-decade insurgency for autonomy in Turkey's mainly Kurdish southeast.
Turkey also portrays the Kurds as a pawn of Russia. Relations between the former Cold War rivals hit a low last year after Turkey shot down a Russian warplane over Syria.
Turkey now accuses Russia of deliberately targeting civilians in Syria, including hospitals struck this week, in what it calls a "war crime" to depopulate territory ahead of a government advance. Moscow denies this and accuses Turkey of covertly supporting Syrian jihadist militant groups.
The United States, which has supported the Kurdish fighters elsewhere in battle against Islamic State, has called for the YPG to stop actions that would heighten friction in northern Syria. It has also urged Ankara to stop shelling YPG positions.
Washington has seen no evidence that the YPG are cooperating with the Russians, U.S. State Department spokesman Mark Toner said at a briefing this week.
نستخدم ملفات الكوكيز لنسهل عليك استخدام مواقعنا الإلكترونية ونكيف المحتوى والإعلانات وفقا لمتطلباتك واحتياجاتك الخاصة، لتوفير ميزات وسائل التواصل الاجتماعية ولتحليل حركة المرور لدينا...اعرف أكثر