U.N. officials in Gaza "were informed by their liaison officers in the Israeli military that the entire population of Gaza north of Wadi Gaza should relocate to southern Gaza within the next 24 hours," U.N. spokesman Stephane Dujarric said in a statement in New York.

"This amounts to approximately 1.1 million people," Dujarric added, or nearly half of Gaza's 2.3 million population. The areas north of Wadi Gaza include Gaza City, the enclave's largest city.

Dujarric said the U.N. "considers it impossible for such a movement to take place without devastating humanitarian consequences."

Israel's military did not immediately provide comment on the warning, which came as it amassed tanks near the Gaza border and pounded the enclave with air strikes.

But appearing to confirm a warning took place, Israel's ambassador to the U.N., Gilad Erdan, said: "The U.N.'s response to Israel's early warning to the residents of Gaza is shameful."

Erdan said the U.N. should focus on condemning Hamas and supporting Israel's right to self-defence.

Salama Marouf, head of the Hamas government media office, said the relocation warning was an attempt by Israel "to broadcast and pass on fake propaganda, aiming to sow confusion among citizens and harm our internal cohesion."

He added: "We urge our citizens not to engage in these attempts".

Hamas' weekend rampage killed 1,300 people and was the worst attack on civilians in Israel's history.

Israel has responded so far by putting Gaza under siege and launching a bombing campaign that destroyed whole neighbourhoods. Gaza authorities said more than 1,500 Palestinians have been killed.

Dujarric said the order by the Israeli military - received by the U.N. in Gaza shortly before midnight on Thursday - also applied to all U.N. staff and those sheltered in U.N. facilities, including schools, health centres and clinics.