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 favor 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.