The US has deployed a truck-mounted missile system into Syria, an official confirmed.
The equipment was supplied to a group of rebels and US military advisers that have repeatedly clashed with government forces.
The deployment raises the stakes in eastern Syria, where Iranian-sponsored pro-government forces have outflanked US advisers and rebels holding the Tanf border crossing to establish their own link to Iraq for the first time in years.
They are now waiting for pro-Iranian forces to link up with them on the Iraqi side of the border, while preparing to march on Islamic State (IS) positions to the north, in the Euphrates River Valley.
Shifting the HIMARS missile system into eastern Syria from Jordan will give the US a precise, long-range weapon to protect its advisers and allies in Tanf, and to attack IS militants further downfield. It has a range of 186 miles.
The deployment also ensures that the military can protect itself from attacks in the area when weather would limit the ability of strike aircraft to reach there in time.
US-backed rebels were advancing north along the Syrian-Iraqi frontier, against IS militants, when pro-government forces cut them off with a flanking manoeuvre last week.
US special forces operators are embedded with the rebels in an advisory capacity, the Pentagon said, though they have fought alongside their allies in defence battles. They hold two outposts in the desert region.
The US central military command has said it remains committed to reaching and defeating the IS group in its strongholds along the Euphrates, beginning with Boukamal, 137 miles north-east of Tanf.
Meanwhile, a UN investigative commission for Syria reported that last month's "de-escalation" agreement has reduced violence in only one of the four zones included in the deal and has not led to greater humanitarian access to besieged areas across the war-torn country.
Underscoring the ongoing violence, a barrage of air strikes and artillery shells pummelled different areas of southern Daraa province on Wednesday, leaving at least eight people dead, including children, first responders and activists said.
A western diplomat said that the US, Russia and Jordan were holding closed door-meetings in Amman to halt the fighting between rebels and the government in southern Syria.
The three states are debating the boundaries of a ceasefire line between the government and rebels in what is hoped to be a comprehensive agreement that would delineate the control of border crossings with Jordan, the diplomat said.
In Geneva, the Independent International Commission of Inquiry told the UN Human Rights Council that fighting around the central province of Homs, near Damascus and in the southern city of Daraa has continued - despite the deal brokered by Russia, Turkey and Iran in May.
Only in the northern Idlib province and western Aleppo has there been "discernible reduction" in violence, said the chair of the UN commission, Paulo Sergio Pinheiro.