Syrian state television reported on Sunday that multiple explosions had been heard inside a US base in the Al-Tanf region near the Iraqi border.
The report was denied, however, by a commander in the US-backed rebel group Maghawir al-Thawra, whose several hundred fighters work with US troops at the Tanf base, who said the blasts were part of joint ground and air exercises that began earlier this week and did not come from inside the base.
The garrison is located in a strategic area near Syria's Tanf border crossing with Iraq at the crossroad of a main Baghdad-Damascus highway, Tehran's main arms supply route by land to Syria and Lebanon's powerful Hezbollah militia.
Several drones attacked the outskirts of the base last October but there were no American casualties, according to US officials.
While it is not common for attacks on the US troops at the outpost, Iranian-backed forces have frequently attacked American troops with drones and rockets in eastern Syria and Iraq.
Russia and the Syrian government have repeatedly called on Washington to pull its troops from the Tanf base, where it has declared a 55 km (35 mile)-radius "deconfliction zone".
The garrison was first set up when Islamic State fighters controlled eastern Syria bordering Iraq, but since the militants were driven out Tanf has assumed a role as part of a US strategy to contain Iran's military build-up in eastern Syria.