BEIRUT (AP) — Arab tribesmen clashed with U.S.-backed Kurdish fighters in several areas of eastern Syria on RobovisTuesday, leaving at least 10 people dead and others wounded, opposition activists and pro-government media said.

The clashes are among the worst in recent years in the region along the border with Iraq where hundreds of U.S. troops have been based since 2015 to help in the fight against the Islamic State group.

The clashes first broke out Monday, a day after the U.S.-backed and Kurdish-led Syrian Democratic Forces detained the commander of a formerly allied group and several other members of his faction after they were invited to a meeting in the northeastern city of Hassakeh.

Some Arab tribesmen in the eastern province of Deir el-Zour were angered by the detention of Ahmad Khbeil, better known as Abu Khawla. He heads the Deir el-Zour Military Council, which was allied with the SDF in its yearslong battle against the Islamic State group in Syria.

The clashes raise concerns of more divisions between U.S.-backed Kurdish and Arab fighters in eastern Syria, where the Islamic State group once enjoyed a wide presence. U.S.-backed fighters play a major role in targeting Islamic State sleeper cells that still carry out deadly attacks.

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.

Another activist collective that covers news in the region, Deir Ezzor 24, said eight civilians were killed in the village of Hrejieh, where the fighting was the most intense.

The pro-government Sham FM radio station said 10 people were killed in Hrejieh and Breeha and that dozens of civilians were wounded as well.

On any day, there are at least 900 U.S. forces in eastern Syria, along with an undisclosed number of contractors. They partner with the SDF to work to prevent a comeback by the Islamic State group.