Syria border war brewing as US boosts anti-IS force

US-backed Syrian fighters say they received new shipments of arms from Washington and its allies, according to Reuters, as the groups face a potential onslaught from pro-Damascus forces looking to open a supply corridor with Iraq.

Jaish Usoud al-Sharqiya (Lions of the East Army) are among the rebel groups who have fought a long and sustained campaign against the Islamic State group in the Syrian Desert region this year, only to see their gains lost to recent regime advances.

Regime militias are just kilometres away from al-Tanf, which is held by US and Jordanian-backed anti-IS forces and an important supply route for forces fighting IS in the south-east Syria regime.

The town also contains a key highway to Iraq, where many Iranian-organised militias bolstering Bashar al-Assad's forces are based.

The regime advance could lead them on a collision course with US and Jordanian-backed Maghawir al-Thawra (Commandos of the Revolution) in al-Tanf, as well as the US forces based there.

US planes have already bombed one pro-regime convoy heading to al-Tanf last month and dropped leaflets on the militias this week warning them against an advance on the supply base, according to opposition media.

The pro-Damascus militias which have been moving closer to Tanf last week, holding manoevers as they bolster their forces.

The US and its Arab allies are wary of the formation of what has been described as a "Shia Crescent" in the Middle East, which would allow Iran to supply its allies and proxies in Iraq, Syria and Lebanon overland.

Iraq's Hashd al-Shaabi militias - supported by Iran - based near Mosul advances to the Syrian border this week, whcih is controlled by pro-US Kurdish militias and IS on the other side of the wire.

The US has supplied its first shipment of arms to Kurdish-led Syrian Democratic Forces, who are perched on the edge of Raqqa and who Washington is said to be counting on to capture large parts of eastern Syria.

It comes after a visit to Saudi Arabia - a key rebel backer, and regional foe of Tehran - by US President Donald Trump, who used the opportunity to warn Iran against further intervention in the region.