Hundreds of thousands of civilians are still inside west Mosul, caught up in deadly fighting between the Islamic State group and Iraqi forces who are backed by coalition air strikes in the battle to retake the area from the jihadists.
West Mosul is both smaller and more densely populated than the city's east, meaning that this stage of the battle poses a greater danger to civilians than those that came before.
"If those innocents were killed, it was an unintentional accident of war," he said.
Townsend described the fighting in the narrow streets of Iraq's second city as the "most significant urban combat" since World War II and "probably the toughest and most brutal close-quarters combat that I have experienced in my 35 years of service."
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The coalition had previously said it carried out a strike on March 17 in an area of west Mosul in which civilian casualties were reported, and that it had opened an investigation.
UN rights office spokeswoman Ravina Shamdasani said yesterday that more than 300 civilians have been killed in west Mosul since February 17.
IS has targeted civilians and used them as human shields, while strikes by anti-IS forces have also left civilians dead.
UN human rights chief Zeid Ra'ad Al Hussein called on Iraqi and US-led coalition forces "to undertake an urgent review of tactics to ensure that the impact on civilians is reduced to an absolute minimum."
IS's "strategy of using children, men and women to shield themselves from attack is cowardly and disgraceful. It breaches the most basic standards of human dignity and morality," he said.