Fighters from the Syrian Democratic Forces looking toward the northern town of Tabqa, Syria. US-backed Syrian forces have launched their attack on the Islamic State group's de facto capital of Raqqa, in northern Syria. Credit:AP

It is unclear how many troops were killed or wounded in the bombing run and what legal rationale might cover the US-led forces continued strikes on Syrian government forces operating within their borders.

On May 18, US forces struck a convoy of pro-Syrian government forces that were driving in the direction of the US base at Tanf. After warnings, both through the hotline and overhead passes, US aircraft struck a number of their vehicles. That convoy, roughly 29 kilometres from the US outpost, came to halt. In response, the US-led coalition added additional "combat power" to help defend the Tanf base and dropped more than 90,000 leaflets telling the Syrian forces to leave.

"We are not increasing our role in the Syrian civil war, but we will defend our troops," Defence Secretary Jim Mattis told reporters following the May strike. "And that is a coalition element made up of more than just US troops, and so we will defend ourselves [if] people take aggressive steps against us."

The Tanf outpost, located near the Syrian, Iraq and Jordanian border, has quietly become a flashpoint in the six-year old Syrian conflict in recent weeks. Iranian-backed Shiite militias loyal to Assad, along with Syrian government forces, are making a concerted push to recapture territory from the Islamic State in the area in a likely attempt to secure the border and the desert highway that runs into Iraq.