It seems a US convoy was stopped at a Syrian army checkpoint near the city of Quamishli where it was refused unobstructed passage. US troops tried to negotiate but the situation became tense when civilians started to surround the convoy. Some started throwing stones, one of them managed to climb on one American vehicle and ripped the American flag off it.
Both Syrian and US personnel called for assistance. A Russian patrol turned up and Russian servicemen tried to calm the crowd down while a pair of F-15 also arrived on the site, dropping flares.
Out of nowhere, a Syrian individual, dressed in civilian clothes, opened fire on the US convoy, starting a firefight. A Syrian citizen was subsequently killed (probably the gunman).
The American convoy turned around and headed back to its base.
Qamishli is situated on the Turkish border. The city remained in Syrian hands throughout the Syrian conflict. The countryside around the city, however, has long been under Kurdish control. US forces were for a long time free to roam around.
However, following the Turkish Military operation Peace Spring (9th of October 2019), Kurdish militias vacated the area, replaced by more Syrian soldiers. The whole area has a mixed Arab/Kurdish population. This partially explains why the US convoy was threatened, yesterday. The same situation explains why Russian convoys in the same area are sometimes stoned.
Another partial explanation is that the departure of the SDF, the partial drawdown of US troops from the province and the arrival of Russian and more Syrian troops in the area has most probably emboldened the local Arab population as well as the SAA.
This incident certainly highlights how complex the whole situation in the country is.