WASHINGTON — As the long-anticipated Iraqi military campaign to take Mosul from the Islamic State (IS) got underway this week, humanitarian aid groups warned of the plight of up to 1.5 million civilian inhabitants of the city, hundreds of thousands of whom may be displaced by fighting in the coming weeks. The United Nations said the scale of displacement that could be triggered by Mosul military operations, with estimates ranging from 200,000 to 1 million people, may pose one of the single biggest and most complex humanitarian challenges it has ever faced.

“Depending on the intensity and scope of the fighting, as many as 1 million people may be forced to flee their homes in a worst-case scenario,” Stephen O’Brien, the United Nations undersecretary-general for humanitarian affairs and emergency relief, said Oct. 16.

Preparations have been underway since February for the Mosul campaign, and the United Nations said it had shelters prepared to house 60,000 people, while construction of additional sites to accommodate up to 250,000 people is taking place.

But with Iraq already housing 3.3 million internally displaced people before the Mosul operation began, the UN had received only approximately 58% of its 2016 Iraq funding request of $861 million. “Despite generous new funding pledges, the US $284 million Mosul flash appeal … is only just over half funded,” the UN’s Office for the Coordinator of Humanitarian Affairs (OCHA) said. “As a result, key components of the response, including emergency camps, are critically underfunded.”