Iraq’s military says a blast that killed nearly 200 civilians was caused by an Islamic State booby trap, not a U.S. airstrike.

The incident is being investigated by the Pentagon, who confirmed Friday that the U.S. struck the blast area in an airstrike requested by the Iraqi Security Forces.

Circumstances surrounding the blast are murky, with one Iraqi Counter-terrorism commander saying the strike was called in on an ISIS car bomb. “When the (vehicle) was struck, it exploded, destroying one or two of the houses next to where families were hiding,” he told CNN. Another Iraqi officer told The New York Times the blast was caused by an airstrike called on ISIS snipers on the roof of a building, that Iraqi Security Forces did not know contained civilians.

Col. John Thomas clarified to the New York Times Friday that the U.S. was trying to investigate whether the blast was caused by a U.S. airstrike or an ISIS booby trap, but the U.S. has not corroborated the Iraqi military’s claim.

Whatever the cause of the blast, the incident highlights the presence of hundreds of thousands of civilians who remain inside ISIS-controlled Mosul. ISIS continues to use civilians as human shields, concentrating them in their areas of operations to deter U.S. airstrikes.

The latest phase of the Iraqi Security Force’s advance on western Mosul only amplifies the odds of civilian casualties. Neighborhoods in the western half of the city have narrower streets and are more densely populated, making Iraqi armored Humvees useless against ISIS suicide assailants. Iraqi’s Observatory for Human Rights estimates nearly 4,000 civilians have been killed in the last month, compared to the nearly 2,000 killed in operations to retake the eastern part of the city.

“Any military operation in the world in this type of environment, there’s going to be casualties,” an Iraqi military officer told The Wall Street Journal. “With this type of enemy, that risk goes up dramatically,” he continued.

When the Mosul operations began in mid-November, an Iraqi officer lamented to the Washington Post, “If there were no civilians, we’d just burn it all.”

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