"There was an incursion into the de-confliction zone," coalition spokesman US Army Col. Ryan Dillon told CNN, adding that after the pro-regime troops entered the zone, the US-led coalition used the pre-established de-confliction hotline and communicated the incursion to the Russians. Following that communication, the pro-regime forces withdrew.
But the apparent successful use of the de-confliction channel comes amid new accusations by Russia that the US military is aiding ISIS.
The incident comes the same day as the Russian Ministry of Defense said the US was the "the main problem" preventing the defeat of ISIS in Syria," accusing the US military of providing "support and overtures" to ISIS.
The ministry's statement said that ISIS fighters had attacked Syrian regime checkpoints located alongside the highway from Palmyra to Deir Ezzor and that those ISIS fighters launched their attack from the de-confliction zone surrounding At Tanf.
"The above mentioned terrorist attacks have the only common feature -- they were conducted from 50-kilometer zone round the settlement of al-Tanf, (near the) the Syrian-Jordan boarder. That is the site where the 'US military mission' is deployed," the Russian Ministry of Defense said.
The Russians added that its air forces were "ready" to strike the area around At Tanf to eliminate the groups that attacked the regime outposts.
Dillon rejected the Russian claims, saying the US-led coalition's goals were "to kill ISIS or compel them to surrender," adding that the coalition does not negotiate with the terror group.
He also denied the idea that ISIS fighters were able to operate in the area surrounding At Tanf.
"We are hyper aware of what is inside At Tanf and near our forces" Dillon said, saying the coalition would go after them and kill ISIS fighters wherever they were.
Coalition officials have said that US troops continue to conduct patrols in the area and use the base to train local anti-ISIS fighters. A group of such fighters left At Tanf in July due to the coalition's insistence that they only fight ISIS and not the regime of Assad, opting to leave and conduct independent operations against the regime.
CNN has previously reported that pro-regime forces had established a number of outposts and checkpoints in the area just outside the de-confliction" zone surrounding At Tanf, a buffer area that 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 US-backed local fighters to defect to the regime and conducting probing activities around the base and nearby coalition combat outposts.
The coalition recently shuttered and razed on of its outposts near At Tanf but denied that its closure was part of any arrangement with Russia or the regime.
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.
Russian accusations of US support for ISIS comes as the US has boosted its surveillance in Syria following a Russian airstrike on US-backed forces and amid Russian threats against US troops and their local allies.
The Russian Ministry of Defense issued a warning last month that its forces would retaliate against the US-backed Syrian Democratic Forces and their US military advisers if any of their forces accompanying Syrian regime troops come under attack. The ministry issued a statement saying that already "the Syrian Democratic Forces and US Special Operations Forces have twice targeted Syrian troops with mortars and rocket artillery."