Clashes broke out Monday between two U.S.-backed groups in eastern Syria, leaving three gunmen dead and raising tensions in the region where hundreds of American troops are deployed, opposition activists said.

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

Monday’s clashes came a day after the U.S.-backed and Kurdish-led Syrian Democratic Forces detained the commander a formerly allied group and several other members of his faction after they were invited to a meeting in the northeastern city of Hassakeh on Sunday.

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The SDF did not confirm 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. SDF officials did not immediately respond to questions by The Associated Press about the arrest.

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Three people were killed when clashes broke out between U.S.-backed groups in eastern Syria.   (AP Photo)

Rami Abdurrahman, head of the Britain-based Syrian Observatory for Human Rights, an opposition war monitor, said that the SDF was concerned that Khbeil was opening links with the Syrian government as well as Turkey, whose troops have carried out several incursions targeting Kurdish fighters in north Syria since 2016.

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Khbeil's arrest could increase tension between Kurds and Arabs because most of his supporters who were also detained belong to a powerful tribe in eastern Syria. In July, a clash between the two sides left at least one Arab fighter dead.

On Monday, several opposition activists reported clashes between the sides in villages in Deir el-Zour. The Observatory and Omar Abu Layla, a Europe-based activist who follows events in the region, said three members of the Deir el-Zour Military Council were killed. Other activists said that angry Arab tribesmen cut roads in the province in protest against the SDF.

The Observatory also reported that members of the Kurdish police force known as Asayesh stormed the offices of Baz news network, an activist collective, and detained five citizen journalists including the head of the network. The Observatory said the network is funded by Khbeil.

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Baz news network said in a statement that Kurdish fighters detained its journalists, confiscated equipment and took control of its offices.

On any given 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 on preventing a comeback by the Islamic State group.