Iran has helped to train and organise thousands of Shi'ite militia fighters from Iraq, Afghanistan and Pakistan in the Syrian conflict.
Tlass Salameh, head of the Jaish Usoud al-Sharqiya, a Syrian rebel group backed by the Central Intelligence Agency programme, said: "There has been an increase in the support".
The southeast of the Syrian desert, known as the Badia, has become an important front in Syria's civil war between President Bashar al-Assad, backed by Iran and Shi'ite militias, and rebels seeking to oust him.
Armed groups under the umbrella of the majority-Shiite Muslim Popular Mobilization Forces (PMF), also called Al-Hashd Al-Shaabi, have strategically pushed westward, battling the remains of ISIS' self-proclaimed Sunni caliphate along the Syrian border.
"We struck what appeared to be a firebase they were constructing, they had a bulldozer and a front-end loader used to create berms", Davis said, describing the USA coalition strike against pro-regime militias on May 18.
The PMU said that the Wadi al-Midar and Taru villages [in northwestern Iraq] were liberated from ISIS, adding that 21 ISIS fighters were killed in the operation backed by the Iraqi air force.
Two armoured vehicles newly delivered to the Tanf garrison were shown in photos sent to Reuters from a rebel source.
In a written response to emailed questions from Reuters, a spokesman for the US -led coalition did not say if coalition support to Maghawir al-Thawra had increased.
The massing of hundreds of pro-regime forces outside of Tanf was described as risky and a "direct threat" to coalition forces, by CENTCOM officials. "Pro-regime patrols and the continued armed and hostile presence of forces inside the. zone is unacceptable and threatening to coalition forces". "Coalition forces are prepared to defend themselves if pro-regime forces refuse to vacate the deconfliction zone".
US jets this week dropped leaflets on pro-government forces instructing them to pull out of the Tanf area to the Zaza junction further from the border.
According to documents recently obtained by Syrian news website Zaman al-Wasl, there are now 88,733 foreign Shia militiamen fighting for Assad in Syria. "This victory will also be an important incentive for the Syrian Arab Army to secure the entire border from the Syrian side", he added. Iran is not only an ally of the USA -backed Iraqi government, but also of the Russia-backed Syrian government of President Bashar al-Assad, who faces a six-year insurgency from ISIS and various other rebel groups, some of which are backed by the U.S.
"The regime and its allies are racing to establish an east-west "Shiite axis" from Iran to Lebanon, and the United States [is] seemingly looking to cement a north-south "Sunni axis" from the Gulf states and Jordan to Turkey", Balanche said. "The Americans will not be allowed to control the border", its leader, Hadi al-Amiri, told al-Mayadeen TV.