US air power has been used in Syria for the first time to attack forces other than Islamic State (IS), a Pentagon spokesman has confirmed.
Capt Jeff Davis said "defensive support fire" was provided last Friday.
This was during clashes between the Free Syrian Army, fighting alongside US-trained members of the New Syria Force, and suspected al-Nusra fighters.
Capt Davis said the US would provide defensive fire support to the NSF "no matter whom they came up against".
Many will see this as "mission creep" as the administration of President Barack Obama has always insisted that the battle inside Syria is against IS alone, the BBC's Gary O'Donoghue at the Pentagon reports.
Speaking to reporters on Monday, Capt Davis said the US had provided fire support to the NSF against the opponents who looked "an awful lot like al-Nusra". However, he added that US officials could not be certain.