At least 105 civilians were killed when a U.S. airstrike hit an explosive-laden building in western Mosul that ISIS was using as a sniper nest, the U.S. acknowledged Thursday.

Axar.az reports citing Yeni Safak.

The announcement follows a coalition investigation into the fatal March 17 strike in the al-Jadida neighborhood in Mosul that prompted global concern.

Brig. Gen. Matthew Isler told reporters 101 civilians were killed in the targeted building, as well as four other victims in a neighboring structure, when a U.S.-fired GBU-38 set off at least 1,000 pounds of TNT-equivalent explosives covertly placed at the rear of the building's second floor by ISIS.

The GBU-38 is a 500-pound "lightweight" precision bomb.

Isler said it alone could not have toppled the building where, unbeknownst to the coalition or Iraqi forces,101 civilians were taking shelter, and that post-blast analysis detected chemical residues common to ISIS explosives, but not the GBU-38.

"The overall structural damage and the crater at the rear of the building were not caused by the low explosive weight of a single GBU-38," he said. "Instead, structural engineering analysis determined that the ISIS-emplaced explosive material conservatively contained more than four times the net explosive weight of the GBU-38."

Iraqi forces requested the U.S.-led coalition strike the building after Iraqi troops took fire from snipers holed up there.

At the time of the strike neither the U.S.-led coalition nor Iraqi forces were aware civilians were inside the building, the coalition stressed.

The GBU-38 was intended to destroy only the top floor of the building, but when it set off the cache of explosives laden with accelerants the resultant damage was far greater, Isler said.

Isler could not pinpoint when ISIS moved the explosives into the building, saying there was a two-day gap in effective surveillance directly leading up to the March 17 strike.

"Just by being in that area and seeing the weather, ISIS probably has had a, and we know they have a good sense, of when coalition force ISR is effective," he said, referring to intelligence, surveillance and reconnaissance. "They would have had the opportunity to move a significant amount of explosive material."

There was a separate entrance from the street to the second floor via a staircase, Isler said. No civilians were allowed on the second floor where ISIS's snipers were encamped, he added.