Battle for Mosul: US investigating deadly air strike
The US military has acknowledged that aircraft of the coalition fighting so-called Islamic State (IS) in Iraq hit a location in west Mosul where dozens of civilians were reportedly killed.
It says an investigation is under way.
Meanwhile, thousands of Mosul residents have fled the areas held by IS, in fear of US-led air strikes and fierce fighting by Iraqi ground troops.
Residents say IS is using civilians as human shields, hiding in houses and forcing young men to fight.
The US Central Command said the planes acted at the request of Iraqi security forces. It did not name which country's aircraft carried out the attack.
In its statement, it said "an initial review of strike data" indicated that an air strike on 17 March was carried out in western Mosul "at the location corresponding to allegations of civilian casualties".
The coalition "takes all allegations of civilian casualties seriously and a formal Civilian Casualty Credibility Assessment has been opened to determine the facts surrounding the strike", it went on.
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The details of what happened are still unclear, but reports have suggested the strikes killed more than 100 people. The number of victims could not be independently confirmed.
'Hundreds of bodies'
Reporters in the Jadideh neighbourhood of west Mosul said they saw 50 bodies being pulled out of rubble on Friday, after the buildings were razed in attacks earlier in March.
One resident who escaped Mosul said hundreds of bodies remained under rubble, the BBC's Middle East editor Jeremy Bowen reports. She lost nine members of her family.
If confirmed, the series of air strikes would rank among the highest civilian death tolls in a US air operation since 2003, when the US led the invasion of Iraq, the New York Times reports.
The Iraqi army, which launched an offensive to recapture Mosul from IS in October, is now closing in on the densely populated Old City. A large plume of smoke could be seen hanging over the area on Saturday as air strikes continued.
In neighbouring Syria, where the US-led coalition is also fighting IS, at least 33 people were killed earlier this week in an air strike, with activists blaming coalition jets.
Iraq's Vice President Osama al-Nujaifi, himself from Mosul, said a "humanitarian catastrophe" was unfolding on account of the excessive use of fire power.
The UN was "stunned" at the loss of civilian lives, said its humanitarian co-ordinator in Iraq, Lise Grande.
"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.
The number of people to have fled west Mosul since Iraqi forces launched their offensive last month now stands above 200,000, according to the Iraqi ministry of migration said on Saturday.
Iraqi security forces, with the help of Kurdish Peshmerga fighters, Sunni Arab tribesmen and Shia militiamen, retook eastern Mosul in January.
The UN estimates that 400,000 Iraqi civilians are trapped in the Old City of Mosul. US officials believe there are about 2,000 IS fighters left in western Mosul.