"Despite previous warnings, pro-regime forces entered the agreed-upon de-confliction zone with a tank, artillery, anti-aircraft weapons, armed technical vehicles and more than 60 soldiers posing a threat to coalition and partner forces based at the At Tanf Garrison," a coalition statement said.
After several warnings over the de-confliction channel, the coalition destroyed "two artillery pieces, an anti-aircraft weapon, and damaged a tank."
Al-Tanf on the Syrian-Jordanian border, which Russian warplanes bombed a year ago in two successive airstrikes, is manned by US and British Special Forces that are advising an anti-ISIS Syrian rebel group known as Maghawir al-Thawra, or the Commandos of the Revolution.
Pro-Assad Shia militias backed by Iran have historically operated in that area.
US officials publicly labeled the forces that entered the de-confliction zone as pro-regime but sources told CNN that they were largely Iranian-backed Shia militias.
The entire incident lasted about six hours, according to a US defense official, who said it began when pro-regime forces inside the de-confliction zone came under attack by what were believed to be ISIS fighters.
At this point, the US contacted Russian forces through established de-confliction communication channels to tell all parties to leave the area. But instead, the pro-regime forces begin bringing in additional fighters and equipment -- a move that US forces deemed a threat to American troops and prompted the bombing, the official said.
Last week, the coalition said that a relatively "small element" of pro-regime forces continued to hold their position inside the "de-confliction" zone surrounding the coalition training base.
Coalition spokesperson, Col. Ryan Dillon, told reporters last week that the pro-regime forces inside the zone represented an "armed and hostile presence" and "they pose a threat to us in that particular location."
"We are prepared to defend ourselves if pro-regime forces refuse to vacate the de-confliction zone," Dillon added.
On May 18th, CNN reported a similar incident when US-led coalition aircraft bombed pro-Syrian regime forces that "were advancing well inside an established de-confliction zone."
During that encounter, two US aircraft were dispatched as a "show of force" to get a convoy of 20 pro-regime vehicles to turn around, a US official said last month.
The official statement said that the vehicles did not stop despite the show of force and warning shots.
A second US defense official said, at the time, that an airstrike hit the convoy after the vehicles continued toward the base, striking an enemy tank and a bulldozer or front loader-type vehicle.
The convoy appears to have consisted not of regular Syrian army soldiers but of international Shia militiamen.