The attack on late Wednesday night involved both kamikaze drones and rockets, US Central Command (CENTCOM) said, attributing the attack to "Iran-backed militias," as it has other similar attacks on its garrisons in Syria and Iraq in recent years.
An eyewitness to the attack told the BBC there were “some coming from [the] Iraq-Syria border," which is just a few miles to the southeast of at-Tanf.
"The al-Tanf garrison area was subjected to a deliberate and coordinated attack," White House Press Secretary Jen Psaki told reporters on Friday, noting she had no reports of deaths or injuries by any US personnel at the base.
"Of course, we always reserve the right to respond," she added.
Photos of the damage emerged on social media on Thursday that were subsequently confirmed by CENTCOM to be of the base. They show several structures crumpled, scorched and otherwise damaged by the attack, including one photo showing the inside of a gym.
The remote desert base, located along the M2 Baghdad-Damascus highway near Syria's border with Iraq and Jordan, has been held by American troops since early 2016, when the base was used to train so-called "moderate rebels" fighting against the Syrian government of President Bashar al-Assad.
The attack on at-Tanf, which hasn't been claimed by any group, came in the wake of Israeli airstrikes against several locations near Palmyra, which the Russian Ministry of Defense said had been launched from the at-Tanf area.
However, the command of the Allied Operations Room of Syria, the pro-Syrian, anti-Daesh* alliance of forces that includes advisers from Hezbollah and Iran's Islamic Revolutionary Guard Corps (IRGC), pledged after that attack that it would respond in kind.
The Biden administration has launched several other responsive airstrikes against Shiite militias in Syria since taking office in January, claiming the right of self-defense after rocket attacks were launched against US forces or bases housing US forces. However, the American response didn't always land on militias responsible for the attacks.
Neither the presence of American troops on Syrian soil, nor attacks on forces or installations on Syrian soil, have the permission of the Syrian government of President Bashar al-Assad.