MOSUL, Iraq - The US-led anti-jihadist coalition said on Tuesday it "probably" played a role in civilian casualties in west Mosul, as the United Nations called for greater efforts to protect civilians.

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 airstrikes 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."