"They were building a fighting position" about 55 kilometers from a USA -coalition base close to At Tanf, where advisers train members of the Syrian Democratic Forces and Syrian Arab Coalition, the second official said.
As long as the USA keeps backing anti-regime insurgents in Syria and is willing to attack pro-regime forces as part of that support, there will be a danger that an incident like this could lead to a larger conflict with the Syrian government and its patrons.
Coalition warplanes targeted a convoy of Syrian government forces and Iranian-backed militia close to the Iraq border on Thursday.
The US-led Inherent Resolve coalition - which includes British forces - confirmed that it had attacked Syrian troops who were pushing back US-supported Free Syrian Army (FSA) insurgents towards the southern at-Tanf border crossing with Iraq.
Muzahem al Saloum, from the Maghawir al Thwra group, told Reuters that the jets struck after some rebel forces clashed with Syrian and Iranian militias after they had advanced to about 17 miles (27 km) from the base.
U.S. Defense Secretary Jim Mattis said America's role in Syria's conflict was unchanged.
Syrian ambassador to the United Nations and chief negotiator Bashar al-Ja'afari said the U.S. air strike had been raised at the talks.
"The new USA strike on government troops in Syria is a continuation of Washington's non-constructive policy", said Leonid Slutsky, the head of the International Affairs Committee of the State Duma, the lower chamber of the Russian parliament. The last encounter between the groups occurred last month when Syrian troops passed near a special operations group in the Kurdish areas, but that remained peaceful.
A commander in the alliance fighting in support of Assad called the strike a "warning raid", adding it was not aimed at causing casualties but instead stopping the advance.
Unlike the USA strike against the al-Shayrat air base in March, which the Trump administration ordered in retaliation for a regime chemical weapons attack, Thursday's strike seemed created to protect the United States forces based at Tanf.
The April strikes were ordered in retaliation for a chemical weapons attack that Washington blamed on Damascus, and were described as a one-off measure to deter any future chemical weapons use.