America brings in the big guns to its southern Syria base

The US has hurried in truck-mounted long-range missiles to southern Syria, as pro-Damascus militias close in on a base used by American special forces and their Syrian allies.

High Mobility Artillery Rocket Systems (HIMARS) have been moved to al-Tanf base in Syria - close to the Iraqi border - where US special forces are training Syrian fighters to take on the Islamic State group.

The artillery provides the base with more protection from Iranian-backed militias that have closed in on the area and give the US-backed Maghawir al-Thawra group a better chance of taking more territory from IS, Reuters reported.

A spokesperson from the group said that the US special forces are playing a bigger role in the fight against IS in southern Syria.

Increased firepower in the area should also deter pro-Assad forces from launching an attack on al-Tanf and US-backed forces.

Washington is said to have set up a second base close to al-Tanf, although a spokesperson for the anti-IS force denied these reports.

"We have that garrison in al-Tanf that is a temporary base and location for us to train our partner forces to fight to defeat IS, but that is the only base in southern Syria or location where we have coalition forces," Colonel Ryan Dillon told Reuters.

Abu al-Atheer, a spokesperson for Maghawir al-Thawra, told the agency that more weapons were being delivered to his Syrian fighters and that US special forces have been on patrol 100km outside Tanf.

"The (new) base was being studied for months but now it's an official base. It has been built and expanded and God willing will be in the next few days like the Tanf base," he told Reuters.

It comes after a surprise offensive by Syrian regime forces through the desert to the Iraqi border, cutting the rebel group off what appeared to be their planned objective of Deir az-Zor city in eastern Syria.

The US could be concerned that plans to capture the IS-held eastern desert region of Syria could be scuppered by the latest regime advance and opens up the possibility of fresh clashes between the two groups.

Israel will also be alarmed at the potential of a direct land route opening from Iran to Hizballah in Lebanon.

There have been continued clashes between a convoy of regime forces heading towards Tanf from the north and the US-backed force.

US forces bombed regime militias near al-Tanf for a third time last week after Iranian-backed militias crept to a restricted area.

A US F-15 also shot down an Shahed 129 drone in the area, after the Iranian aircraft dropped bombs on anti-IS coalition forces and allied Syrian fighters.