The end game against ISIS is getting messier

Reuters:

Western-backed Syrian rebels holding a strategic swathe of the desert southeast stretching to the Iraqi border said they came under major attack on Tuesday from government forces and allied Iranian-backed militias backed by Russian air power.

They said hundreds of troops with dozens of armored vehicles including tanks had surged into the Bir Qassab area some 75 km (45 miles) southeast of Damascus towards the Badia region that skirts the borders with Jordan and Iraq.

Bir Qassab straddles the route to the eastern suburbs of Damascus, near the Dumeir air base, that is also a key rebel supply line towards areas they control further southeast.

Bir Qassab fell to Free Syrian Army (FSA) rebels after it was abandoned a few months ago by Islamic State (IS) militants beating an eastward retreat to reinforce their urban bastion, Raqqa, against a U.S.-backed coalition offensive, and Deir al Zor province against a thrust by Syrian government forces.

Bir Qassab had given Islamic State a springboard for attacks on territory just to the east of Damascus and a base for maintaining their grip on large swathes of the Badia region.

"The (Syrian) regime and militia ground attack started this dawn and our forces are holding on to their positions," said Saad al Haj, spokesman for Osoud al Sharqiya, one of the largest rebel groups operating in the area.

"With (the help of) intensive Russian bombing they are trying to advance but we are repelling them," al-Haj added.

President Bashar al-Assad's forces, aided by Iranian-backed militias, have engaged in a race with FSA rebels in recent weeks to seize areas in the southeastern desert vacated by retreating Islamic State insurgents.
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With ISIS under heavy pressure from US-backed forces, the Assad regime in Syria is trying to grab real estate for what remains of Syria after ISIS is defeated.  I suspect this action was in a region outside the US zone of operation for its aircraft.

But once Raqqa falls that zone is also likely to be contested:

Once Islamic State stronghold of Raqqa falls, U.S. will be on collision course with Syria and Iran

Trump administration officials are planning for what they see as the next stage of the war, a complex fight that will bring them into direct conflict with Syrian government and Iranian forces contesting control of a vast desert stretch in the eastern part of Syria.

It is already happening in the lead up to the fall of Raqqa.