"The United Nations considers it impossible for such a movement to take place without devastating humanitarian consequences," Stephane Dujarric, spokesperson for the UN secretary-general, said on Thursday.

He told AFP that the UN "strongly appeals for any such order, if confirmed, to be rescinded avoiding what could transform what is already a tragedy into a calamitous situation."

Israel's army said it has called for all Gaza City residents to evacuate their homes and head south of the territory "for their safety".

Dujarric said the army's evacuation order applies to all UN staff and those sheltering in UN facilities -- including schools, health centers and clinics.

The UN agency for Palestinian refugees (UNRWA) is sheltering more than 60 percent of the 423,000 people displaced in recent days in the Gaza Strip.

It was not immediately clear how many people were currently located north of Wadi Gaza.

Israel has been reeling since an October 7 attack, when Hamas militants killed more than 1,200 people and took about 150 captive.

In a grisly retaliation, Israel has rained air and artillery strikes on Gaza -- a densely populated enclave of 2.3 million people -- flattening buildings and killing more than 1,400 people, many of them civilians.

A Security Council meeting on the conflict has been called for Friday.

*This story was edited by Ahram Online