The United Nations urged Israel to rescind its call for 1.1 million residents of northern Gaza to relocate to the south of the territory within 24 hours, amid massive bombardments in retaliation for Hamas's attacks.

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He told AFP that the UN "strongly appeals for any such order, if confirmed, to be rescinded avoiding what could transform what is already a tragedy into a calamitous situation."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

"The IDF (Israeli Defense Forces) calls for the evacuation of all civilians of Gaza City from their homes southwards for their own safety and protection and move to the area south of the Wadi Gaza as shown on the map," the army said in a statement.

"In the following days, the IDF will continue to operate significantly in Gaza City and make extensive efforts to avoid harming civilians."

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 gunmen swept into small towns, kibbutzim and a music festival, killing more than 1,200 people and taking about 150 hostages.

In 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.

Israel's Ambassador to the UN Gilad Erdan said the UN's reponse to the evacuation call was "shameful."

"For many years, the UN has turned a blind eye to the arming of Hamas and its use of the civilian population and civilian infrastructure in the Gaza Strip as a hiding place for its weapons and murder," Erdan said in a note from his office to AFP. 

"Now, instead of standing by Israel...it preaches to Israel".  

"It is better for the UN to focus now on returning the hostages, condemning Hamas, and supporting Israel's right to defend itself," he said.