BEIRUT (AP) – The United States military called for an end to days of fighting between rival U.S.-backed groups in east Syria Thursday, warning it may help the resurgence of the Islamic State group.

The fighting that broke out Monday—and left at least 40 people dead and dozens wounded in the eastern oil-rich province of Deir el-Zour—was the worst in years. East Syria is where hundreds of U.S. troops have been based since 2015 to help battle IS militants.

The clashes pitted members of the Kurdish-led Syrian Democratic Forces against its former ally the Arab-led Deir el-Zour Military Council and some regional Arab tribesmen who had sided with them.

The Combined Joint Task Force – Operation Inherent Resolve said in a statement that it continues to closely monitor events in northeast Syria adding that “we remain focused on working with the Syrian Democratic Forces to ensure the enduring defeat of Daesh, in support of regional security and stability.” It used the Arabic acronym to refer to IS.

Sporadic clashes occurred in different parts of the area along the border with Iraq on Thursday and both sides were reportedly bringing in reinforcements.

On Wednesday, the SDF and the council jointly said that Ahmad Khbeil, better known as Abu Khawla, would no longer command the Deir el-Zour Military Council. He and four other militia leaders were dismissed over their alleged involvement in “multiple crimes and violations,” including drug trafficking.

Mr. Khbeil was also removed over “coordination with external entities hostile to the revolution,” apparently a reference to his purported contacts with the Syrian government in Damascus and its Iranian and Russian allies.

The latest round of clashes raised concerns of more divisions among the SDF and its allies in eastern Syria, where IS had once controlled large swaths of territory and where the extremist group’s militants still stage occasional attacks.

“Distractions from this critical work create instability and increase the risk of Daesh resurgence,” the U.S. military said. “The violence in northeast Syria must cease, and the effort returned to creating peace and stability in northeast Syria, free from the threat of Daesh.”

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 an IS comeback.