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American aircraft carried out retaliatory strikes in Syria against forces loyal to President Bashar al-Assad on Wednesday, U.S. officials said.

Syria's state-run SANA news agency reported that the bombing left dozens of people dead and wounded. NBC News could not immediately confirm that account.

No embedded U.S. troops were believed to have been wounded or killed in an earlier attack targeting a headquarters of the American-backed and Kurdish-led Syrian Democratic Forces, U.S. officials told Reuters on condition of anonymity.

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The SDF is an alliance of militias in northern and eastern Syria.

The U.S.-led coalition fighting Islamic State militants did not say whether any pro-Syrian fighters were killed in the retaliatory strike.

The coalition described the attack on the SDF headquarters as "unprovoked," but offered little other information in a terse statement confirming the incident.

SANA reported that the overnight strikes hit tribal fighters who back Assad in Khusham, a town in oil-rich Deir el-Zour. SANA said the local fighters were battling Islamic State militants and the SDF.

The Syrian army is backed by Iranian-backed militias and Russian forces.