Witnesses and local sources said at least 40 fighters from both sides and another 15 civilians have been killed in battles raging since Sunday in a string of towns in a strategic oil belt in the heart of Arab tribal territory, east of the Euphrates River controlled by the Syrian Democratic Forces (SDF).

Fighting erupted last Sunday after the SDF arrested Ahmad Al Khubail, alias Abu Khawla, who headed its Deir al Zor Council. He also was leading Arab commander with the SDF, a U.S. formed alliance of militias whose backbone is the Kurdish YPG militia.

In its first comment on the situation, the SDF issued a statement saying Abu Khawla was arrested and dismissed from his post for complicity in multiple crimes from drug trafficking to failing to handle the Islamic State threat in the province.

Residents said the SDF has pushed new reinforcements as it shelled several towns and villages where armed Arab clans burnt tyres, ambushed vehicles and shelled SDF positions.

Tribal fighters took over several checkpoints and attacked patrols In several towns, including Shuhail near the al Omar oil field close to where U.S. troops are stationed.

Tribal figures and residents said mounting unrest triggered by the arrest of the Abu Khawla exposed deep anger at the Kurdish-led militia that controls a predominately Arab population in the oil-rich province.

Arab residents with poor living conditions complain the Kurdish-led administration does not give them their share of the oil wealth.

The Syrian YPG Kurdish leadership, whom the Pentagon equips with weapons, have long denied discriminating against Arabs and says it seeks to redress injustice against Kurds that denied them their culture before Syria's conflict began in 2011.

The YPG has been the main U.S. partner in Syria in ousting Islamic State from Deir al-Zor and beating the Syrian army and its Russian backers to lay hands on some of Syria’s biggest oil fields.