The US-led coalition accidentally struck the Kurdish-led Syrian Democratic Forces (SDF) in Tabqa, killing 18 moderate Syrian rebels fighting against Daesh. The incident occurred on April 11.
"The target location was actually a forward Syrian Democratic Forces fighting position."
The coalition is probing the cause of the incident, according to the release.
Tabqa, which is believed to be the last stronghold of Daesh is located some 40 kilometers (25 miles) away from Raqqa. Taking the city is part of the operation to liberate Raqqa with the offensive against Tabqa having started on March 21.
In November 2016, the SDF, which predominately consists of Kurdish fighters but also includes members of the Syrian Arab Coalition, launched an operation called the Rage of Euphrates to seize control over Syrian Raqqa, the de facto Daesh capital.