The U.S. military said a misdirected airstrike killed 18 soldiers from the allied Syrian Democratic Forces (SDF) in northern Syria on Tuesday.

U.S. Central Command (CENTCOM), which oversees U.S. forces in the Middle East, said in a statement Thursday that allied Syrian forces on the ground had requested a strike against a position south of the town of Tabqa, Syria. The ground forces believed the position was controlled by fighters from the Islamic State of Iraq and Syria (ISIS), the military said.

Instead, the targeted location was held by fighters with the SDF.

“The Coalition’s deepest condolences go out to the members of the SDF and their families,” CENTCOM said, calling the incident “tragic.”

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The military said the coalition is investigating the mix up and “will implement appropriate safeguards to prevent similar incidents in the future,” CENTCOM said.

The town of Tabqah is approximately 30 miles from Raqqa, the self-declared ISIS capital and the largest city still under the militant group’s control. Anti-ISIS forces on the ground are preparing to mount an offensive against Raqqa, supported by U.S. and coalition air power as well as U.S. special forces.

In March, a monitoring group in Syria said dozens of civilians had been killed by a U.S. strike outside Raqqa. The military said it had no evidence of civilians being killed.