US air power has been used for the first time to defend US-trained forces fighting in Syria, 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".
Meanwhile, Russian Foreign Minister Sergei Lavrov described as "counter-productive" US comments that Washington could take extra measures to defend the US-trained Syrian rebels.
Speaking at a news conference in Qatar, Mr Lavrov said this "could complicate the task of fighting terrorism" in Syria.
Russia is a key ally of Syria's embattled President Bashar al-Assad.
Speaking to reporters on Monday, Capt Davis said about 50 fighters had attacked members of the FSA who were co-located with forces from the NSF.
He said the US had provided fire support against the opponents who looked "an awful lot like al-Nusra". However, he added that US officials could not be certain.
He did not provide further details about where the clashes took place.
Capt Davis also said the US would provide defensive fire support to the NSF against "broader threats". He did not deny that this could mean US forces could come in to contact with fighters loyal to President Assad.
Al-Nusra Front - Syria's al-Qaeda affiliate - is one of the most powerful insurgent groups in Syria.
Last week, it claimed to have captured a number of rebel fighters who were trained by the US.
A statement from the militant group warned others against taking part in "the American project".
Reports said the leader of the so-called Division 30 and other members had been taken - but this has been denied by the Pentagon.
Al-Nusra Front also attacked Division 30's base in northern Syria on Friday.
Five of the group's fighters were killed and several more wounded in the clashes, Division 30 said in a statement.
Al-Nusra said some of its fighters were killed in US air strikes against their positions near the city of Aleppo.
Its statement did not say how many Division 30 fighters had been captured, or when they were taken.
Division 30 were trained under a US-led programme to build a moderate force to fight the Islamic State group.
The Syrian conflict began as an uprising against the government, but that has since splintered, pitting rebel groups fighting President Assad's forces against one another.