US-led coalition aircraft bombed forces allied to the Syrian government on Thursday, after they advanced too close to a base where Western special forces are training rebel fighters.

American jets were understood to have struck a convoy of 27 tanks as they moved to within 15 miles of a coalition garrison in al-Tanf, a border crossing point to Iraq in southern Syria, marking the most direct clash between coalition forces and fighters with Bashar al-Assad's regime.

Syrian, Iranian and allied Lebanese Hizbollah forces have in recent days moved surface-to-air missiles closer to the eastern frontline with the moderate Free Syrian Army (FSA) units, in an apparent warning to the US-led coalition, which flies sorties against Islamic State of Iraq and the Levant (Isil) in the area.

"A convoy going down the road didn't respond to numerous ways for it to be warned off from getting too close to coalition forces in al-Tanf," a US official confirmed, saying they breaching the so-called deconfliction zone radius around the encampment.

US aircraft then attempted to buzz the regime, but when the convoy did not turn around, they conducted a strike against some of the vehicles.

"Then there was finally a strike against the lead portion of that movement,"he official said.