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The top American general in Mosul says the initial military assessment into civilian casualties allegedly caused by a March 17 U.S. airstrike shows the U.S. "probably had a role in these casualties," according to the top American general there.

Lt. Gen. Steve Townsend said Tuesday there is "at least a fair chance" that American forces had a role in killing civilians. Unconfirmed reports suggest the death toll could range from 100 to 200.

"If the U.S. did this," said Townsend, it was an "unintentional accident of war."

Officials had previously said initial analysis indicated coalition forces struck ISIS fighters and equipment on March 17 in western Mosul, and confirmed there was an assessment into possible civilian deaths in a destroyed building. U.S. Central Command (CENTCOM) said the aircraft acted at the request of Iraqi security forces.

Townsend said Tuesday that the U.S. still doesn’t know whether the civilians who died in the building were placed there by ISIS, adding that the Iraqis believe ISIS gathered the civilians intentionally.

Townsend would not go into specifics about why the U.S. "probably had a role" in the deaths, other than to say the U.S. did conduct a strike in the area at the time.

But he also said that the fact that the building collapsed actually argues against U.S. culpability, saying that the U.S. strike used a munition that should not have collapsed the building.

Related: US Investigates Whether Mosul Airstrike Killed 200 Civilians

Townsend said again and again that the U.S. does everything possible to prevent civilian casualties. "We have never targeted civilians, not once," he said, adding later, "I’m not targeting civilians. ISIS is."

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He denied that any recent change in rules of engagement has caused an increase in civilian casualties. Civilian deaths in western Mosul are "fairly predictable," he said, describing the battle there as the toughest, most brutal phase of the war, with close combat more difficult than he has seen in more than three decades in uniform.

Related: US Airstrike Hit Mosul Location Where Scores of Civilians Were Killed

The U.S. is also investigating a recent strike on an Iraqi school that may have resulted in civilian casualties. Townsend said that the early indications are that it was a "clean strike," and that there is "no corroborating evidence" of civilians killed. He said coalition forces struck the approximately 30 ISIS fighters believed to be there and he thinks the investigation will show the allegations are unfounded.

Iraq's vice president, who is from Mosul, said a change in the U.S. rules of engagement — which are meant to minimize civilian casualties in Iraq and Syria — had resulted in the "martyrdom of hundreds of civilians" in the March 17 strike.

A senior U.S. defense official denied Saturday that there had been any change in the rules of engagement.

The United Nations expressed profound concern on Saturday over the reports.

"We are stunned by this terrible loss of life," Lise Grande, the humanitarian coordinator for Iraq, said in a statement.

U.S. defense officials have acknowledged that coalition aircraft did strike a location in the Iraqi city of Mosul where scores of civilians were reportedly killed last week.

Officials at United States Central Command (CENTCOM) said initial analysis indicates coalition forces struck ISIS fighters and equipment on March 17 in western Mosul. Unconfirmed reports from different sources suggest the numbers of those killed ranges from 100-200 people.

CENTCOM said the aircraft acted at the request of Iraqi security forces.

Officials had previously confirmed an investigation was underway as to whether the strike was executed by the U.S. rather than by other coalition members, or even by ISIS.

It comes as Iraqi vice president Osama al-Nujaifi, who is from Mosul, described the incident as a "humanitarian catastrophe" blaming the U.S.-led coalition airstrikes and the excessive use of force by federal police forces.

He called for an emergency session of the Iraqi parliament to discuss the catastrophe and to begin a parliamentary investigation into its cause.

Residents carry the body of several people killed during fights between Iraq security forces and Islamic State on the western side of Mosul. Felipe Dana / AP

The strike comes in the same month as two other high-profile strikes in Syria, where the coalition is also fighting ISIS. In total, the three strikes have resulted in unconfirmed reports of upwards of 350 civilian casualties.

The vice president said a change in the rules of engagement — which are meant to minimize civilian casualties in Iraq and Syria — had resulted in the "martyrdom of hundreds of civilians".

A senior U.S. defense official denied on Saturday that there had been any change in the rules of engagement. The CENTCOM statement said "our goal has always been for zero civilian casualties."