The United Nations says Israel has told it that 1.1 million Palestinians in Gaza should relocate to the region’s south within the next 24 hours.

The announcement, on Friday afternoon (AEDT), came as Israel’s military chief said “now is the time for war” as his country amassed tanks near the Gaza Strip ahead of a planned ground invasion to annihilate the Palestinian militant Hamas group that was behind last weekend’s deadly attacks.

Israel has pounded Gaza from the air since the weekend incursions, the deadliest by Palestinian militants in its history, and has been preparing for a ground invasion.

The UN said Israel’s military had warned that everyone living north of Wadi Gaza should relocate to southern Gaza – 1.1 million people, or about half the population of the entire Gaza Strip.

The order came just before midnight local time.

“The United Nations considers it impossible for such a movement to take place without devastating humanitarian consequences,” UN spokesman Stephane Dujarric said.

“The United Nations strongly appeals for any such order, if confirmed, to be rescinded avoiding what could transform what is already a tragedy into a calamitous situation.”

Friday’s warning came as undreds of Australians prepared to board repatriation flights out of Israel as a ground war looms in Gaza.

Prime Minister Anthony Albanese said 1600 people had registered in Israel or the West Bank, including 19 in Gaza, for repatriation in what was an “extraordinary logistical exercise”.

A Qantas flight was due to depart Ben Gurion International Airport on Friday night for London. A connecting flight is planned from London to Sydney via Singapore on Tuesday.

The initial flight is expected to land in Sydney on Wednesday morning, and a second Qantas flight will leave Tel Aviv on Sunday.

The government confirmed on Friday at least two further charter flights would take Australians from Tel Aviv to Dubai.

“We’re doing all that we can – this is an extraordinary logistical exercise while a war is going on,” Mr Albanese said.

Qantas said 900 crew volunteered for the flights, far exceeding the 70 required.

Virgin and Qatar Airways also offered to help Australians trying to leave.

The situation for 19 Australians trapped in Gaza, including a family of four from Adelaide, is worsening as they face an imminent Israeli invasion and a lack of food, water and electricity.

“Our beloved children are living in a constant state of fear and, with each passing day, the availability of food and clean water dwindles further,” a member of the family said in a letter to the federal government.

“This might be my last message to anyone.

“Please can the government negotiate an exit strategy through Egypt?”

Foreign Minister Penny Wong urged Australians wanting to leave not to delay.

“This is a very difficult situation … the United States and other parties are seeking to establish humanitarian access, humanitarian corridors, and Australia supports those efforts,” she said in Adelaide.

Wong said some contact had been made with the family but could not provide further detail.

Australian Friends of Palestine Association spokesman Mike Khizam called on the government to do more to secure their escape.

“The situation there is horrific now, but we’re expecting it to get much, much worse,” he said.

“We need the Australian government to balance its concerns both for Palestinian civilians and Israeli civilians.”

An estimated 10,000 Australian citizens are in Israel, including dual citizens and tourists.

Land border entries to Jordan remain open but Australians have been advised to confirm the status of the crossings prior to departure.

The Israeli death toll since last weekend’s attacks by the terror group Hamas has risen to more than 1300. Most were civilians gunned down in their homes, on the streets and at a music festival.

Israel says 97 Israeli and foreign hostages were taken back to Gaza.

Gaza authorities say more than 1400 Palestinians have been killed and more than 6000 wounded in the retaliatory strikes.

Among those killed in Israel was Australian grandmother Galit Carbone, 66, who died at the hands of Hamas militants who attacked her village near the Gaza border.

-with AAP