The U.S.-led coalition fighting the Islamic State in Iraq and Syria (ISIS) accidentally killed 18 fighters from the coalition’s main Syrian partner on the ground, the coalition said Thursday.

A “misdirected” airstrike hit a fighting position of the Syrian Democratic Forces (SDF) south of Tabqa on Tuesday, according to a coalition statement.

The SDF is a coalition of Arab and Kurdish fighters and is considered by the United States to be the most effective local force fighting against ISIS.

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The SDF, backed by coalition forces in the air and on the ground, has been fighting for control of the city of Tabqa, as well as the nearby strategic Tabqa Dam, as it works to isolate the de facto ISIS capital of Raqqa.

Tuesday’s strike was requested by “partnered forces,” according to the coalition. They had identified the target as an ISIS fighting position, the statement said, but the target was actually a forward SDF fighting position.

“The coalition’s deepest condolences go out to the members of the SDF and their families,” the coalition statement said. “The coalition is in close contact with our SDF partners who have expressed a strong desire to remain focused on the fight against ISIS despite this tragic incident.”

The coalition is assessing the cause of the incident and will implement “appropriate safeguards” to prevent future incidents, the statement added.

The incident could reignite questions about the process for calling in airstrikes that were raised after a spate of reported civilian casualties in Iraq and Syria last month.

The battlefield in Syria has grown even more tense in the past week, since the United States struck a Syrian airfield in retaliation for a chemical weapons attack U.S. officials say was carried out by Syrian regime forces.

A spokesman for the coalition said Wednesday that it has pulled back on airstrikes against ISIS in the wake of the airfield bombing.