The HIMARS was deployed close to the Al-Tanf coalition training base, a hotspot for clashes between the US and Syrian government forces
The US has moved its High Mobility Artillery Rocket System (HIMARS) into southern Syria from Jordan for the first time, according to defence officials.
According to CNN, the deployment will position it near the US coalition training base at Al-Tanf, which has been a flash point for clashes between the US and forces loyal to the Syrian government.
The HIMARS is a truck-mounted system which can fire missiles as far as 300km and will heavily boost US military power in southern Syria.
In May, the US-led coalition struck a convoy of military vehicles belonging to Shia militias loyal to the Syrian government near Al-Tanf.
According to a US defence official, the pro-Assad militia convoy was heading to a remote coalition garrison near the Jordanian border.
The US official also said the coalition air strike did not directly target the Syrian army but Shia militias who support the Syrian government which was operating in the area.
"A convoy going down the road didn’t respond to numerous ways for it to be warned off from getting too close to coalition forces in al-Tanf," the official told AFP on condition of anonymity.
Although this marks the first HIMARS deployment in southern Syria, a US official told CNN that the HIMARS had previously been used to aid the Syrian Democratic Forces (SDF) in northern Syria during their campaign against the Islamic State group.
The HIMARS has also struck IS targets in Syria from firing positions in Turkey and Jordan and has been used in Iraq to target IS there.
Russia said on Saturday it had told the United States it was unacceptable for Washington to strike pro-government forces in Syria following the Al-Tanf attacks last month.
Russian foreign minister, Sergei Lavrov, relayed the message to US Secretary of State Rex Tillerson in a phone call on Saturday initiated by the US side, the Russian Foreign Ministry said in a statement.
Russia said at the time that the US action would hamper efforts to find a political solution to the conflict and had violated the sovereignty of Syria, Russia's closest Middle East ally.
"Lavrov expressed his categorical disagreement with the US strikes on pro-government forces and called on him to take concrete measures to prevent similar incidents in future," the ministry said.
The two men had also exchanged assessments of the situation in Syria, it added, and confirmed their desire to step up co-operation to try to end the conflict there.
The ministry said Lavrov and Tillerson had also discussed the need to try to mend the rift between Qatar and other Arab nations through negotiations, and had talked about the state of US-Russia relations and planned meetings between officials from the two countries.
Al-Tanf, on the key highway connecting Damascus with Baghdad, has been the focus of advances by Iran-backed troops loyal to Syrian President Bashar al-Assad. The area, also just northeast of the Jordanian border, is used by US-backed forces as a training and staging area for attacks against IS.