US Central Command (CentCom) said it had targeted two snipers from so-called Islamic State (IS) with what it called a "precision-guided munition".
However, the strike detonated explosives that militants had placed in the building, CentCom said.
Civilians sheltering in the lower floors were killed when it collapsed.
CentCom said the death toll included four civilians in another nearby structure.
Eyewitnesses claimed another 36 non-combatants were also in the building, but US authorities said it had "insufficient evidence to determine their status".
