The US military has confirmed its warplanes hit an area of the besieged Iraqi city of Mosul held by the so-called Islamic State group where residents and officials say dozens of civilians have been killed as result of an air strike.
The incident occurred on 17 March but what exactly happened is still unclear as IS militants fight to defend the areas of the city they still control.
Iraqi government forces paused in their push to recapture western Mosul 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".
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 air strikes by Iraqi or US-led coalition forces.
US Central Command, which oversees US military operations in the Middle East, said a review had determined that US-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.
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 other lay buried in the rubble.
The exact cause of the collapses was not clear but a local politician and two residents said on Thursday the air strikes ma yhave detonated an IS truck filled with explosives, destroying buildings in the heavily-populated area.
The speaker of the Iraqi parliament, Salim al-Jabouri, said in a statement: "What's happening in the west part of Mosul is extremely serious and could not be tolerated under any circumstances."
Up to 600,000 civilians are still believed to remain in IS-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.