The Central Command statement said that the target location turned out to be a “fighting position” for the Syrian Democratic Forces, who have been fighting the Islamic State alongside the United States.

It was unclear whether the strike came from an American warplane or one from the other coalition partners.

“The coalition’s deepest condolences go out to the members of the S.D.F. and their families,” Central Command said in the statement, calling the episode “tragic.” Military officials said the cause is being investigated.

As the American-led military campaigns against the Islamic State in Iraq and Syria have intensified in recent months, so, too, have reports of civilian casualties and, now, friendly fire.