BEIRUT (AP) — The U.S. military says a misdirected airstrike this week killed 18 allied fighters battling the Islamic State group in northern Syria.

U.S. Central Command said Thursday that coalition aircraft were given the wrong coordinates by their partner forces, the predominantly-Kurdish Syrian Democratic Forces, for a strike intended to target Islamic State militants south of their Tabqa stronghold — located east of Raqqa.

The strike hit an SDF position instead, killing 18 fighters. Central Command says the incident occurred on Tuesday.

The SDF, with the help of air and ground support from the U.S.-led coalition, has surrounded Tabqa.

Several nations have lent their air power to the coalition to defeat the Islamic State group. It is not clear which air force was behind the strike.