“Most of the killed belong to militias loyal to the Syrian regime and are not Syrians,” Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights told dpa.
He added that there are a number of wounded, meaning the death toll will likely go up.
The Observatory said that four military vehicles carrying regime forces and their allies were destroyed in the strike.
Coalition planes struck pro-regime forces that were advancing inside a protected “deconfliction zone” north-west of the al-Tanf area, the military alliance said in a statement. It added that the advance had posed a threat to US and partner forces in the area.
Coalition forces demonstrated a show of force and fired warning shots before the airstrike took place, according to the statement. The coalition said its forces have been operating in al-Tanf for many months, training and advising “partner forces” engaged in the fight against the Islamic State extremist militia.
“The target of the airstrike was clearly affiliated with the regime,” an official at the US Department of Defence told dpa, without giving details.
There was no immediate official comment from Damascus.
Also on Thursday, Islamic State militants shelled a regime-controlled area in the city of Deir al-Zour in eastern Syria, killing 14 people and wounding 21 others, the Observatory said.
The shelling targeted a wedding ceremony in the neighbourhood of Harabish, which is located close to Deir al-Zour military airport.
Most of the city of Deir al-zour is under the control of the Islamic State, but a small part of it remains in the hands of regime forces.
At least 15 civilians were among 52 people also killed in fighting in the central province of Hama Thursday, following a major offensive by Islamic State, according to the Observatory, which relies on a network of activists inside war-torn Syria.
The attack targeted Hama’s regime-held villages of Aqareb and al-Mabujah.
After heavy fighting, regime forces managed late Thursday to retrieve the village of Aqareb, a few hours after Islamic State militants took control of it.
Syria’s state news agency SANA said 52 Aqareb residents were killed by Islamic State militants in the attack.
The agency added that government forces had repulsed the offensive and killed an unspecified number of the extremists.
The US is currently leading an air campaign in Syria targeting extremist groups, mainly Islamic State.
In recent months, the extremist group has suffered military setbacks and lost territory in the country.
On April 7, the US launched dozens of missiles against the Shayrat airbase in central Syria. An earlier chemical weapons attack that killed more than 80 civilians is thought to have been launched from there.