Opponents of Syria’s President Bashar al-Assad are showing signs of splintering along a deep regional fault line, with Arabs and Turks uneasy about a military offensive last month by Syrian Kurds, who overran four towns in the country’s north.

The attacks marked the first time since the 17-month-old uprising began that Kurdish fighters had joined in military action against Assad’s forces. But the Kurdish muscle-flexing has rattled groups such as the Arab-led Free Syrian Army, which until now has played the leading role in the upheaval, and it has unsettled neighboring Turkey, whose animosity toward Assad is surpassed only by apprehension about the Kurds’ broader ambitions in the region.