A U.S. strike targeting Daesh terrorists in a Mosul building in Iraq in March killed at least 105 civilians when the blast caused Daesh weapons to explode, a U.S. general said Thursday.

"The secondary explosion triggered a rapid failure of the structure which killed the two [Daesh] snipers, 101 civilians sheltered in the bottom floors of the structure and four civilians in the neighboring structure to the west," U.S. Air Force Brigadier General Matt Isler said of the March 17 incident.

Isler said the United States and nearby Iraqi forces did not know there were civilians in the building or that it had been rigged with explosives.

He added that 101 civilians inside the building were killed, four civilians were killed in nearby, and 36 civilians were still not accounted for.

Local officials and eyewitnesses have said as many as 240 people may have died in the strike.

The probe found that the U.S. bomb triggered secondary explosions from devices clandestinely planted there by Daesh. And the military says the secondary blasts caused the concrete building to collapse.

It was likely the largest single incident of civilian deaths since the U.S. air campaign against Daesh began in 2014.

The deaths represent about a quarter of all civilian deaths since the U.S. air campaign began.

Prior to this investigation, the U.S.-led coalition against Daesh said that at least 352 civilians have been killed in strikes it carried out in Iraq and Syria since 2014. That estimate is far lower than those provided by outside groups.