The pro-government forces had been backed by artillery, tanks and other heavy weaponry when they attacked an SDF headquarters "eight kilometres [5 miles] east of the agreed-upon Euphrates River de-confliction line", United States officials said.
One Syrian rebel soldier was wounded but no USA forces were killed or wounded, according to NPR's Tom Bowman, who is reporting from elsewhere in Syria.
According to reports a headquarters of the Syrian Democratic Forces came under fire from artillery and tanks.
A USA military official told CNN that the area where the us coalition and SDF compound is located contains lucrative oilfields and the Syrian government forces "were likely seeking to seize oilfields in Khusham that had been a major source of revenue for [Islamic State] from 2014 to 2017".
Following the siege on Raqqa, Defense Secretary Jim Mattis said that U.S. forces would focus on diplomacy and that they "won't just walk away" from efforts to stabilize the region.
The attack on the SDF headquarters east of the Euphrates River was well coordinated and involved T-54 and T-72 tanks, as well as artillery.More news: Jerome Powell Takes Oath as Fed Chief
A USA official told CNN that the attackers crossed the Euphrates River and were armed with artillery and other weapons.
After the attack the coalition "conducted strikes against attacking forces to repel the act of aggression". USA troops were there at the time, but none of them were hurt.
The US-led coalition had previously alerted Russian officials about the presence of SDF forces in nearby Khusham after seeing a slow build-up of pro-government forces during the last week.
USA officials said it was not immediately clear whether the attacking forces were part of the the Syrian military or belonged to foreign militias or other units who fight in support of the government. One SDF member was injured.
With the defeat of Islamic State in most of Syria the coalition and the Syrian regime share a long "de-confliction" line mostly along the Euphrates river.
The officials decline to identify details of the attacking forces, which are suspected to be Syrian or from one of a number of allied militia units, including from Iraq and Lebanon, that are backing President Bashar Al Assad in the country's seven-year civil war.