The U.S military confirmed on Saturday that U.S. warplanes had hit an Islamic State-held area of the besieged Iraqi city of Mosul where residents and officials say dozens of civilians have been killed as result of an airstrike. 

The strike occurred on March 17, but what exactly happened is still unclear as ISIS militants fight to defend the areas of the city they still control. Reports have indicated that the airstrikes allegedly killed anywhere from 100 to 200 civilians in western Mosul, where U.S.-backed government troops are fighting ISIS.

Iraqi government forces paused in their push to recapture western Mosul on Saturday because of the high rate of civilian casualties, a security forces spokesman said, a move apparently motivated by the incident.

The United Nations also expressed its profound concern, saying it was "stunned by this terrible loss of life."

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Iraqi civilians flee the city of Mosul as Iraqi forces advance in their fighting against ISIS jihadists on Thursday. (Ahmad Gharabli/AFP/Getty Images)

Initial reports from residents and Iraqi officials in the past week said dozens of people had been killed or wounded in 
Mosul's al-Jadidah district after airstrikes by Iraqi or U.S.-led coalition forces. 

U.S. Central Command, which oversees U.S. military Middle East operations, said on Saturday that a review had determined that U.S.-led coalition aircraft had struck Islamic State fighters and equipment "at the location corresponding to allegations of civilian casualties."

It was investigating to determine the facts and the validity of reports of civilian casualties, it said. 

Iraqi VP blames coalition, Federal Police

Reports on the numbers of civilian dead and wounded have varied, but Civil Defence chief Brigadier Mohammed al-Jawari told reporters on Thursday that rescue teams had recovered 40 bodies from collapsed buildings. Many others lay buried in the rubble.

The exact cause of the collapses was not clear but a local politician and two residents said the airstrikes may have detonated an ISIS truck filled with explosives, destroying buildings in the heavily populated area.

In tweets published on his official account, speaker of the Iraqi parliament Salim al-Jabouri said "we realize the huge responsibility the liberating forces shoulder" and call on them to "spare no effort to save the civilians."

In a statement issued on his website, Vice President Osama al-Nujaifi, himself from Mosul, described the incident as a
"humanitarian catastrophe," blaming the U.S.-led coalition airstrikes and excessive use of force by militarized Federal Police forces. Al-Nujaifi put the number of civilians killed at "hundreds." 

He called for an emergency session of parliament and an immediate investigation into the incident.

600K civilians still in western Mosul

Up to 600,000 civilians are still believed to remain in ISIS-held areas of Mosul, complicating the government offensive 
tactically but also politically as the Shia Muslim-led government seeks to avoid alienating people in the mainly Sunni  
city.

Residents escaping besieged western Mosul have told of Iraqi and U.S.-led coalition airstrikes demolishing buildings and killing civilians in several cases.

The insurgents have also used civilians as human shields and opened fire on them as they try to escape ISIS-held neighbourhoods, fleeing residents said.

"The recent high death toll among civilians inside the Old City forced us to halt operations to review our plans," a Federal Police spokesman said on Saturday. "It's a time for weighing new offensive plans and tactics. No combat operations are to go on."

The U.S.-backed offensive to drive the group Islamic State in Iraq and Syria out of Mosul, now in its sixth month, has recaptured the entire eastern side of Mosul and about half of the west.

But advances have stuttered in the last two weeks as fighting enters the narrow alleys of the Old City, home to the al-Nuri mosque where ISIS leader Abu Bakr al-Baghdadi declared a caliphate spanning large areas of Iraq and Syria in 2014.

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The Yabasat neighbourhood of western Mosul has largely been destroyed by the fighting. About 600,000 people remain in the areas of west Mosul held by ISIS militants, including 400,000 civilians in the Old City who are living under siege-like conditions, the UN said. (Ahmad al-Rubaye/AFP/Getty Images)

"We need to make sure that taking out [ISIS] from the Old City will not cost unwanted high casualties among civilians. We need surgical accurate operations to target terrorists without causing collateral damage among residents," the Federal Police spokesman said.

A U.S. deputy commanding general for the coalition told Reuters on Friday that the solution could lie in a change of tactics. The Iraqi military is assessing opening up another front and isolating the Old City, where the militants have put up fierce resistance, U.S. Army Brigadier General John Richardson said.

Fleeing residents have described grim living conditions inside the ISIS-held parts of Mosul, saying there was no running water or electricity and no food coming in. 

But families are streaming out of the northern city, Iraq's second largest, in their thousands each day, headed for cold, crowded camps or to stay with relatives. Hunger and fighting are making life unbearable inside.

The Iraqi Observatory for Human Rights said that since the campaign on western Mosul began on Feb. 19, unconfirmed reports said nearly 700 civilians had been killed by government and coalition airstrikes or Islamic State actions.

The militants have used car bombs, snipers and mortar fire to counter the offensive. They have also stationed themselves in homes belonging to Mosul residents to fire at Iraqi troops, often drawing air or artillery strikes that have killed civilians.

The United Nations' Grande said civilians were at extreme risk as the fighting in Mosul intensified and all sides must to do their utmost to avoid such casualties.

"International humanitarian law is clear. Parties to the conflict — all parties — are obliged to do everything possible to protect civilians. This means that combatants cannot use people as human shields and cannot imperil lives through indiscriminate use of fire-power," she said in a statement. 

With files from The Associated Press