Damascus, May 19 (IANS): The US launched a direct airstrike against pro-Syrian forces who advanced inside a "de-conflicted zone" even though Russia urged them not to, the Washington-led coalition said.

There was no immediate word on casualties in Thursday's strike northwest of the coalition base at Al Tanf, NBC News reported.

The airstrike appears to be the first publicly known direct confrontation between US forces and fighters loyal to Syrian President Bashar al-Assad.

While US forces have engaged with Islamic State (IS) militants they have not previously been reported to have directly engaged with elements of the Syrian regime or militias allied with it.

The coalition said Thursday's airstrike was launched because the Syrian forces posed a direct threat to US and allied rebel forces by advancing into the de-confliction zone near Al Tanf despite "apparent attempts" by their Russian backers to dissuade them, as well as a "show of force" from coalition aircraft and the firing of warning shots.

The region around Al Tanf, near the border with Jordan and Iraq, is considered a de-conflicted zone under an agreement between the US and Russia, the coalition added.