As this remote corner of northeastern Syria fast slides out of government control, many Syrians are bracing for what they fear will be another war, between the relatively moderate fighters who first took up arms against the government and the Islamist extremists who emerged more recently with the muscle and firepower to drive the rebel advance. …

Yet even as the regime continues to hold out, schisms are emerging among rebel groups over ideology, the shape of a future Syrian state and control of the significant resources concentrated in this long-neglected but crucial corner of the country.

“Fighting is unavoidable,” said Abu Mansour, a commander with the rebel Free Syrian Army’s Farouq Brigades, whose men clashed last month with those of the extremist Jabhat al-Nusra movement in the border town of Tal Abiyad, one of several instances in which the tensions have erupted into violence. “If it doesn’t happen today, it will happen tomorrow.”

Jabhat al-Nusra, the group designated a terrorist organization by the United States because of its suspected ties to al-Qaeda, is among several groups advancing in the region, but it is emerging as the most divisive and the strongest.