Regional reports have tallied at least 13 to as many as 25 people, both militants and civilians, have been killed.

It happened Wednesday in Deir Ezzor province, which for years has had some 1,000 or more US soldiers occupying what is Syria's only oil and gas rich province. The US has trained and supported Kurkish-led "Syrian Democratic Forces" while also cutting off Damascus and most of the Syrian population from its own resources. 

Al Jazeera reports of the Wednesday violence, "Clashes in eastern Syria between Arab tribal fighters and US-backed Kurdish-led fighters have left several people, including civilians, dead and others wounded, opposition activists and pro-government media have said."

It's being called the worst violence there since 2015, when there were still fierce counter-terror operations against ISIS. 

According to more details: 

The clashes first broke out on Monday, a day after the US-backed, Kurdish-led Syrian Democratic Forces (SDF) detained the commander and several members of the Deir Az Zor Military Council, a group that had been allied with the SDF, at a meeting they invited them to in the northeastern city of Hassakeh.

The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, reported that 10 Arab tribesmen and three SDF fighters were killed in clashes in the villages of Hrejieh and Breeha.