An errant airstrike by a U.S.-backed coalition has killed 18 allied soldiers who were fighting the Islamic State militant group in northern Syria.
U.S. Central Command, which directs U.S. forces in the Middle East, said Thursday allied forces members inadvertently carried out the attack Tuesday south of Tabqa, a strategically important town near the IS stronghold of Raqqa.
Raqqa is the self-declared capital of IS and is the largest Syrian city under the group's control.
Central Command said the predominately Kurdish Syrian Democratic Forces (SDF) gave coalition aircraft pilots the wrong coordinates, resulting in a strike that hit an SDF position instead.
Several countries are providing air support to the coalition in the battle against IS. It is unclear which air forces carried out the strike.
In a statement, the coalition expressed its "deepest condolences" to SDF members and their families.
"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 said, using an acronym for Islamic State.
The statement added "appropriate safeguards" would be implemented to prevent similar incidents from occurring.
A monitoring group in Syria said last month dozens of civilians had been killed in a U.S. attack outside Raqqa. The military said it had no proof of civilians being killed.