US boosts 'combat power' in south Syria to face Iran-backed militias

The remarks by a Baghdad-based spokesman for the US-led coalition battling IS were the latest sign of tension in the region, where the United States has forces at a base near the Syrian town of Tanf supporting local fighters.

"We have increased our presence and our footprint and prepared for any threat that is presented by the pro-regime forces," said the US army spokesman Ryan Dillon on Thursday, referring to Iran-backed forces supporting the Syrian president, Bashar al-Assad.

Dillon estimated that a small number of Iran-backed forces had remained inside a so-called "deconfliction" zone meant to ensure the safety of US-led coalition forces since an 18 May US strike on their advancing formation.

Meanwhile, a larger number has been massing directly outside the zone, which was agreed between the US and Russia, which is supporting Assad.

"We see that as a threat," Dillon said.

The US military has also dropped about 90,000 leaflets this week warning the fighters inside the zone to depart, one US official said. 

Reuters previously reported on the leaflet drop, citing Hammurabi Justice, a website linked to US-backed Syrian rebel forces known as the Maghawir al-Thwra group.

Copies of the leaflets provided to Reuters by the Pentagon told the Iran-backed fighters that any movement towards the Tanf garrison "will be seen as hostile intent and we will defend our forces".

"You are within an established deconfliction zone, leave the area immediately," another read.

This southeastern area of the Syrian desert, known as the Badia, has become an important front in Syria's civil war between Assad, backed by Iran and Shia militias, and rebels seeking to oust him.

They are competing to capture land held by IS, which is retreating as it comes under intense attack in Iraq and along Syria's Euphrates basin.

Western-backed Syrian rebels said on Wednesday that Russian jets attacked them as they tried to advance against Iran-backed militias.

US-backed rebels took Tanf from IS last year, and regional intelligence sources say they mean to use it as a launchpad to capture Bukamal, a town on Syria's border with Iraq and an important militant supply route.

The coalition's presence in Tanf, on the Damascus-Baghdad highway, was also meant to stop Iran-backed groups from opening an overland route between Iraq and Syria, the sources say.

Damascus has declared the Badia and Deir Ezzor priorities in its campaign to re-establish control over Syria, which has been shattered by six years of war that have killed hundreds of thousands of people.