A United Nations official is warning that an upcoming military campaign to wrest control of the Iraqi city of Mosul from the Islamic State group could affect as many as 1.5 million civilians, making it one of the world's largest humanitarian crises.

Lise Grande, the U.N. humanitarian coordinator for Iraq, said Wednesday that military operations to retake areas held by the Islamic State group have already forced hundreds of thousands from their homes.

"The impact of the Mosul military campaign on civilians will be devastating," Grande said in statement from Iraq. "Mass casualties among civilians are likely and families trying to flee are expected to be at extreme."

The United Nations is seeking $284 million ahead of the expected operation, U.N. spokesman Farhan Haq said.