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.

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.