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 residents have fled IS-held Mosul, 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.
In a statement, US Central Command said US planes struck an area in west Mosul corresponding to the reports of civilian casualties, at the request of Iraqi security forces.
“Coalition forces comply with the Law of Armed Conflict and take all reasonable precautions during the planning and execution of airstrikes to reduce the risk of harm to civilians,” the statement said.
- More than 180,000 flee west Mosul battle
- Battle for Mosul: The story so far
- ‘Prisoners’ remains’ in Mosul mass grave
‘Hundreds of bodies’
Witnesses and residents say recent air strikes have killed dozens of people, but the number of victims could not be independently confirmed.
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.
The offensive to recapture Mosul from IS began last October, and multiple air strikes were launched on Saturday morning.
The army is now closing in on the densely populated Old City of Mosul in the western half. A large plume of smoke could be seen hanging over the area.
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.
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.
Battle for Mosul: US investigating deadly air strike}