CAIRO — Aid groups are racing to prepare for what they are predicting will be the largest humanitarian crisis in recent memory, with a million people expected to flee the northern Iraqi metropolis of Mosul once the country’s military and its foreign allies launch their offensive to seize the city from Islamic State militants.

The planned operation to take back Iraq’s second-biggest city could trigger “one of the largest man-made disasters” in years, said Bruno Geddo, the United Nations refugee agency representative in Iraq.

The conflict in neighboring Syria has affected more people, leaving 13.5 million in need of humanitarian assistance. But the Mosul offensive is unique in that aid agencies know roughly when it will start and that it will displace a massive number of people at once.

Representatives of the various forces have said the operation will start between mid-October and mid-December, with Turkish President Recep Tayyip Erdogan recently specifying Oct. 19.

The U.N. made an emergency appeal this summer for $284 million to deal with the crisis but so far has received less than half. The United States has pledged $130 million, said Lise Grande, the U.N. humanitarian coordinator for Iraq.

“We’re facing this enormous tsunami coming at us,” Grande said. “We’re racing against the clock to be as ready for it as we can be, knowing that we have funding constraints, capacity constraints, and that we still need to get more supplies into the country.”