Syrian forces fighting the Islamic State called in a U.S. air strike on their own men, resulting in 18 deaths, according to the Pentagon.

The Syrian Democratic Forces (SDF), a U.S.-backed group fighting ISIS in northern and eastern Syria, requested the strike on what they thought was an ISIS position just south of the city of Taqbah April 11.

(This first appeared in The Daily Caller here.)

The SDF are one of the primary coalition partners fighting ISIS in Syria. Approximately 500 U.S. special operations forces are embedded with the SDF and other anti-ISIS groups in Syria. It is unclear if a U.S. operator was embedded with the SDF unit that called in the strike. (RELATED: US Troops Now Standing Toe-To-Toe With ISIS)

“The target location was actually a forward Syrian Democratic Forces fighting position,” said a statement from Combined Joint Task Force Operation Inherent Resolve, the U.S.-led coalition fighting ISIS in Iraq and Syria. “The Coalition’s deepest condolences go out to the members of the SDF and their families.”

The statement added that the coalition is “in close contact” with the SDF, and that the group is still focused on fighting ISIS despite the incident. The coalition plans to investigate the cause of the accidental strike.

Coalition forces in Syria engaged in a siege against ISIS’s de facto capital in November, and are in the midst of retaking the city and surrounding areas. ISIS leaders reportedly fled the city in early March, but 3,000 to 4,000 militants remain.

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