Exploiting the unravelling of Assad's grip in wide swathes of Syria, Kurds have been asserting control in parts of the northeast, bidding for the self-rule and rights denied to their community for decades under Assad and his father before him.
Some fear the increasingly sectarian tinge of the anti-Assad uprising will splinter Syria. But whoever takes charge in the Kurdish plains nudging against Turkey will control a chunk of Syria's estimated 2.5 billion barrels of crude oil reserves.
On Monday, gunfire clattered in the mixed Arab and Kurdish frontier town of Ras al-Ain, which was overrun by the mainly Sunni Muslim Arab fighters on Nov. 8 and bombed by Assad's forces in the days that followed.
Fleeing residents said the fighting was between insurgents of the Free Syrian Army (FSA) and Kurds affiliated with the Democratic Union Party (PYD), a Syrian Kurdish party with links to Kurdish separatist militants in Turkey.
Opposition activists at the Syrian Observatory for Human Rights also reported clashes, with a rebel advance into Syria's Kurdish heartland meeting resistance.
Kurds in the region are suspicious of the Free Syrian Army and whether any post-Assad, Islamist-dominated government would be more accommodating of Syria's largest ethnic minority.
The Observatory said at least four Kurdish militiamen had been wounded in the fighting and that a rebel sniper had shot dead the leader of Ras al-Ain's local Kurdish Council, Abed Khalil.
"The injuries were the result of clashes between the two sides when opposition fighters launched an assault on a checkpoint belonging to the Units for the Protection of the Kurdish People," it said, referring to a PYD-affiliated militia.
Some residents said Kurdish militiamen had attacked a house where FSA fighters were staying, and that there had been protests against the fighters in pro-PYD areas of Ras al-Ain.
The accounts could not be independently verified.
Civilians, laden with belongings, began trickling again through the barbed-wire border fence into Turkey. Thousands have already fled the town, swelling to around 120,000 the number of Syrian refugees sheltering in camps in Turkey.
With its own large Kurdish minority, Turkey is watching closely, worried that the emergence of an autonomous Kurdish region in Syria could further embolden terrorists of the Kurdistan Workers Party (PKK) fighting for autonomy in southeastern Turkey.
More than 40,000 people have been killed in 28 years of fighting between Turkey and the PKK -- designated a terrorist group by Ankara, the United States and European Union.
Fighting surged over the summer, and Ankara has accused its former ally and now adversary Assad of arming the terrorists.