U.S. officials are weighing whether to broaden the air campaign in Syria to strike a militant group that is a rival to the Islamic State and that is poised to take over a strategically vital corridor from Turkey.

Extremists from the al-Qaeda-linked Jabhat al-Nusra group were said Monday to be within a few miles of the Bab ­al-Hawa crossing in northwestern Syria on the Turkish border, one of only two openings through which the moderate Free Syrian Army receives military and humanitarian supplies provided by the United States and other backers.

Over the weekend, rebels said Jabhat al-Nusra forces swept through towns and villages controlled by the Free Syrian Army in Idlib province, west of Aleppo. Rebel groups associated with the Free Syrian Army were routed from their main strongholds, with scores of fighters fleeing toward Turkey or defecting to join the militants, according to opposition activists. […]