The offensive is part of a campaign to recapture full control of the Syrian Badia, where the advance of the regime and Iranian-backed militias southeast of Palmyra has brought them within reach of the Iraqi frontier for the first time in years.
Syrian regime forces and allied Iranian-backed militias backed by Russian air power reportedly launched a major attack on Western-backed rebels in the east of the country.
The operation to capture a strategic swathe of desert that stretches to the Iraqi border started on Tuesday in the Bir Qassab area, some 75 km (45 miles) southeast of Damascus, rebels said.
Hundreds of regime troops with dozens of armoured vehicles including tanks had surged into area as they headed 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, and is a key rebel supply line towards areas they control further southeast.
The area fell to the Free Syrian Army (FSA) after it was abandoned a few months ago by the Daesh terror group.
Daesh had made an eastward retreat to reinforce their urban bastion, Raqqa, against a US-backed coalition offensive, and the oil-rich Deir Al Zor province, which borders Iraq.
"The [Syrian] regime and militia ground attack started this dawn and our forces are holding on to their positions," said Saad al Haj, spokesperson 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," Haj added.
Race to fill vacuum left by Daesh
Bashar al Assad's regime 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 Daesh.
"Backed by Iran, Iraqi Shi'ite groups are spreading across the Badia," said Abu Yaqoub, a commander in the Ahmed Abdo Martyrs Brigade, an FSA rebel group whose fighters were involved in the battles.
The regime and Russian planes had been waging a relentless bombing campaign on Western-vetted rebel hideouts in the region and making steady gains, taking over several areas including the strategic Tal Dakwa mountain range, northeast of Bir Qassab.
This move has also effectively encircled FSA-controlled desert territory stretching to the Jordanian and Iraqi borders and close to the Tanf garrison where US forces are based.
Iranian-backed forces have also been trying to advance towards the base even after repeatedly being bombed by the US-led coalition.
US forces said on Tuesday they had shot down an armed "pro-Syrian regime" drone near the garrison. A Western intelligence source identified the drone as Iranian.
The coalition's presence in Tanf, on the Damascus-Baghdad highway, was meant to stop Iranian-backed groups from opening an overland route between Iraq and Syria, intelligence sources have said.