Rebels say hundreds of US Marines arrived this month in Tanf to join special operations troops already there.
A Syrian rebel commander said on Thursday that rare military exercises with US Marines in southern Syria sent a strong message to Russia and Iran that the Americans and the rebels intend to stay and confront any threats to their presence.
Colonel Muhanad al-Talaa, commander of the Pentagon-backed Maghawir al-Thawra group, told Reuters the eight days of drills that ended this week at the US military outpost in Tanf were the first such exercises with live-fire air and ground assault involving hundreds of US troops and rebel fighters.
"These exercises have a big importance and have beefed up the defences of the area and raised the combat capabilities and morale and that of civilians in the area," Talaa told Reuters by phone from Tanf, near Syria's borders with Jordan and Iraq.
A US military spokesman said the exercises were a show of force and that the Pentagon had notified Moscow through "deconfliction" channels to prevent "miscommunication or escalate tension".
"The exercise was conducted to reinforce our capabilities and ensure we are ready to respond to any threat to our forces within our area of operations," Colonel Sean Ryan told Reuters in an email response to questions.