Local rebel groups told Reuters that the outpost's closure was the result of an arrangement between Russia and the US.
Such a deal would appear to go against what Secretary of Defense James Mattis told reporters at the Pentagon Monday when asked if the US-led coalition was ceding ground in Syria to the regime following reports that regime troops backed by Russian airpower had crossed the Euphrates River.
"We have not ceded any of the ground we have taken to anybody," Mattis said.
But a spokesman for the coalition told CNN that the closure of the outpost was a routine action and not part of any deal with Russia or the Syrian regime.
"Throughout Syria and Iraq, the coalition has established and closed numerous bases, as warranted by the operational situation, in order to ensure we provide effective support to our partner forces. The decision to establish and close temporary bases is determined by operational requirements and the progress of the campaign," US Army Col. Ryan Dillon said.
Dillon also pointed out that "Coalition and partner forces still operate out of At Tanf" and that "Coalition forces continue to conduct patrols out of At Tanf to train and prepare the MaT for counter-ISIS operations."
The MaT refers to the US-backed Maghawir al-Thawra group, the largest anti-ISIS group based at At Tanf. The MaT regularly performs joint patrols with US Special Operations Forces.
A US military official told CNN that while the base was dismantled and razed over the weekend, its closure had been planned weeks ago and was unrelated to recent events on the ground in Syria.
CNN has previously reported that pro-regime forces had established a number of outposts and checkpoints in the area just outside the 55 kilometer "de-confliction" zone surrounding At Tanf, which was negotiated by Russia and the US-led coalition.
US officials have told CNN they believe pro-regime militias backed by Iran are setting up a series of checkpoints as part of an effort to establish a strategic overland route from Iran to the Mediterranean coast and have sought to eject coalition troops from At Tanf to facilitate this goal, seeking to encourage MaT fighters to defect to the regime and conducting probing activities around the base and nearby coalition combat outposts.
The outpost shuttered over the weekend lies to the east of At Tanf, closer to the Iraqi border, an area the regime has long sought to gain control over. It was raised in part to prevent it being used by another military force, a US official said.
The proximity between the two sets of forces led to some clashes in June, including US airstrikes against regime forces who had encroached on the zone and US F-15 jets shooting down two pro-regime drones that were perceived to pose a threat to local allies and their coalition advisers.
There has also been increased proximity between regime forces and fighters from the US backed Syrian Democratic Forces further east.
That proximity likely contributed to Saturday's Russian airstrike that the coalition said wounded SDF fighters while US Special Operations Forces were present.
US-backed Syrian Democratic Forces recently announced their push into Deir Ezzor province and have made major advances along the Khabur River toward the Middle Euphrates River Valley, an area where the US military believes ISIS has relocated much of its leadership and based 5,000 to 10,000 of its fighters.
While regime forces had primarily operated on the western side of the Euphrates River and SDF forces on the eastern side, the Russian Ministry of Defense issued a statement Monday saying Syrian regime troops, backed by Russian airpower, had crossed over to the river's eastern bank near Deir Ezzor city.
The coalition confirmed to CNN that regime and Russian troops had crossed the Euphrates River, bringing them closer to US-backed forces operating in eastern Syria.
"We confirm Russian forces have moved east of the Euphrates River. The Russians are providing air support to Syrian Regime operations to reclaim territorial control over the area around Deir Ezzor," Dillon told CNN Tuesday.
"The coalition's mission in Syria is to defeat ISIS, and US forces will remain in Syria as long as the Syrian Regime is unable to destroy ISIS pockets and prevent the terrorists' resurgence," he added.
But a US military official criticized the regime's efforts fighting ISIS saying that while they might be able to clear the area, they are unlikely to be able to "hold" areas and prevent ISIS's return. The official cited the experience of what happened in Palmyra, Syria, where ISIS after initially being defeated by regime troops, was able to regain much of the territory it had lost as the area changed hands multiple times.