"Coalition service members in an advise, assist, and accompany capacity were co-located with SDF partners during the attack 8 kilometers (4 miles) east of the agreed-upon Euphrates River de-confliction line," read a US statement released late on Wednesday.
Both coalition and "partner forces" retaliated to the "unprovoked attack against well-established Syrian Democratic Forces [SDF] headquarters."
The coalition explained it "conducted strikes against attacking forces to repel the act of aggression."
The coalition claims to support vetted ground partners in Syria to defeat ISIS, the SDF.
The UK-based SOHR added that shelling started on Wednesday between SDF and regime loyalists in the villages of Bukarah, Jadid Akidat, and in Conoco oil field. Then they exchanged machine gun fire.
The Conoco field is named for the US company which discovered the gas reserves in 1968 and built a processing plant. It is on the eastern bank of the Euphrates, opposite of Deir ez-Zor city.
This prompted the coalition actions, and according to SOHR, resulted in the killing of 20 pro-regime forces.
As they were finishing the Raqqa operation in October, the SDF pushed further south into the Middle Euphrates River Valley in Deir ez-Zor governorate.
The river "serves as a demarcation lines between Russian-backed Assad regime forces to the west and our SDF partners to the east," the coalition's press office told Rudaw English in late December.
Russia and Iran have backed the Bashar al-Assad regime in the Syrian civil war, while the United States claims its mission in the country is to support locals to defeat ISIS.
"The Coalition de-conflicts the complex and congested battlespace over eastern Syria with Russian officials on a regular basis and will continue to do so," it previously stated.
The US military and the SDF did not report casuality figures.
The Syrian Arab Army did not immediately release a statement.
There were unverified reports in Arab media of 100 or more Syrian army or loyalist deaths.
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