The United Nations has raised grave concerns about reports of high civilian casualties in the Iraqi city of Mosul.

A senior UN official in Iraq said she was stunned by accounts of "terrible loss of life", after claims that at least 200 people had been killed in an air strike by the US-led coalition.

US warplanes are supporting the Iraqi Army's mission to retake Mosul from the Islamic State (IS).

US media reports say an investigation is under way.

It is not known exactly when the deaths are alleged to have happened.

However, reporters in western Mosul's Jadideh neighbourhood said they saw 50 bodies being pulled out of buildings on Friday, after they were razed in air attacks earlier in March.

Iraqi forces have been waging a months-long offensive to recapture Mosul, the last IS stronghold in Iraq, which has been occupied since 2014.

The New York Times quoted US military officials saying the coalition was investigating reports of civilian deaths from a strike between 17-23 March.

Colonel Joseph Scrocca, a spokesman for the US-led command in Baghdad, said that "the coalition has opened a formal civilian casualty credibility assessment on this allegation" from Mosul.

"This process takes time, though, especially when the date of the alleged strike is in question," he said.

The UN estimates that 400,000 Iraqi civilians are trapped in the Old City of Mosul as government forces battle to re-capture it.

More than 180,000 civilians have fled the west of the city in the past month, amid fears that an additional 320,000 may follow in the coming weeks.

Residents who have managed to flee say the militants are using civilians as human shields, hiding in houses and forcing young men to fight.

US officials believe there are about 2,000 IS fighters left in Mosul.