Nigeria's military has again killed a large number of civilians with an airstrike it says was targeting militants instead, a mistake that has occurred a number of times in recent years and raises questions about the capabilities of a U.S. security partner.

The latest such strike occurred on Sunday, when Amnesty International’s Nigeria office says at least 100 civilians — some of them children — were killed at a market in Tumfa town in northwestern Zamfara state. A Red Cross official in the state confirmed the airstrike to The Associated Press and said “multiple civilians” were killed.

Nigeria ’s military, however, denied killing civilians. Maj. Gen.…