Israel’s military has informed the United Nations that the entire population of northern Gaza should relocate to the southern half of the territory within 24 hours, the U.N. spokesman, Stéphane Dujarric, said late on Thursday night, adding that such a movement — involving over one million people — would lead to “devastating humanitarian consequences.”

The message came just before midnight in Gaza, he said in a statement. The U.N. has “strongly appealed” for the order to be rescinded, the statement said, to avoid making “what is already a tragedy into a calamitous situation.”

“The same order applied to all U.N. staff and those sheltered in U.N. facilities — including schools, health centers and clinics,” Mr. Dujarric said.

Israeli military officers conveyed the information to the leaders of the U.N. Office of the Coordination of Humanitarian Affairs and the Department of Safety and Security in Gaza just before midnight local time on Friday, the U.N. statement said. The U.N. was told that the marker dividing the north from south was Wadi Gaza, the statement said.

The U.N. Security Council is scheduled to hold an emergency meeting on Friday afternoon in a closed consultation format. The Council has not yet issued a statement about the conflict between Israel and Hamas because a statement requires the approval of all 15 members, and there have been divisions on language that would condemn Hamas’s attack but not address Palestinian grievances and the siege of Gaza.