The Pentagon on Thursday admitted to accidentally killing 18 U.S.-backed Syrian fighters on Tuesday during a coalition operation against the Islamic State (ISIS).

“A coalition air strike in support of partnered forces fighting ISIS south of Tabqah, Syria, resulted in 18 Syrian Democratic Forces personnel killed April 11,” reads the statement from U.S. Central Command (CENTCOM).

The strike, it continued, “was requested by the partnered forces, who had identified the target location as an ISIS fighting position. The target location was actually a forward Syrian Democratic Forces fighting position.”

It was not clear if the strike came from a U.S. military plane or that of another coalition member.