Israel’s military told some 1 million Palestinians on Friday to evacuate northern Gaza and head to the southern part of the besieged territory, an unprecedented order applying to almost half the population ahead of an expected ground invasion against the ruling Hamas militant group.

The UN warned that so many people fleeing en masse would be calamitous. Hamas, which staged a shocking and brutal attack on Israel nearly a week ago and has fired thousands of rockets since, dismissed it as a ploy and called on people to stay in their homes.

The evacuation order, which includes Gaza City, home to hundreds of thousands of Palestinians, sparked widespread panic among civilians and aid workers already running from Israeli airstrikes and contending with a total siege and a territory-wide blackout.

“Forget about food, forget about electricity, forget about fuel. The only concern now is just if you’ll make it, if you’re going to live,” said Nebal Farsakh, a spokesperson for the Palestinian Red Crescent in Gaza City, as she broke into heaving sobs.

The war has already claimed over 2,800 lives on both sides and sent tensions soaring across the region. Israel has traded fire in recent days with Lebanon’s Hezbollah militant group, sparking fears of an ever wider conflict, though that frontier is currently calm.

Weekly Muslim prayers brought protests in some nearby countries, and tensions ran high in Jerusalem’s Old City. The Islamic endowment that manages a flashpoint holy site in the city, the Al-Aqsa Mosque compound, said Israeli authorities were barring all Palestinian men under the age of 50 from entering.

Israel has bombarded Gaza round the clock since a weekend attack in which Hamas fighters stormed into the country’s south and massacred hundreds, including killing children in their homes and young people at a music festival.

Militants also snatched some 150 people and dragged them into Gaza.

Hamas said Israel’s bombardment had killed 13 of the hostages, including foreigners. It did not give their nationalities, saying they were killed over the last 24 hours.

Israeli military spokesperson Rear Adm. Daniel Hagari denied that airstrikes killed hostages, telling Al-Jazeera Arabic that “we have our own information and do not believe the lies of Hamas”.

The military urged all civilians in Gaza’s north to move south, according to Hagari — an order that the UN said affects 1.1 million people.

Israel said it needed to target Hamas’s military infrastructure, much of which is buried deep underground. Another spokesperson, Jonathan Conricus, said the military would make “extensive efforts to avoid harming civilians” and that residents would be allowed to return when the war is over.

Hamas militants operate in civilian areas, where Israel has long accused them of using Palestinians as human shields. A mass evacuation of civilians, if carried out, would leave their fighters exposed as never before.

But UN spokesperson Stephane Dujarric said it would be impossible to stage such an evacuation without “devastating humanitarian consequences”. He called on Israel to rescind any such orders, saying they could “transform what is already a tragedy into a calamitous situation”.

Hamas, meanwhile, called on Palestinians to stay in their homes, saying Israel “is trying to create confusion among citizens and harm the cohesion of our internal front”. It called on Palestinians to ignore what it said was ”psychological warfare”.

Gaza’s health ministry said it was not possible to evacuate the many wounded from hospitals, and that hospital staff would not heed the warning.

“We have a duty and a humanitarian mission, and we cannot evacuate hospitals and leave the wounded and sick to die,” spokesperson Ashraf al-Qidra said. In the event of severe Israeli strikes, he said there was simply no other place in the Gaza Strip to take and treat patients.

The UN agency for Palestinian refugees, known as UNRWA, also said it was not evacuating its schools, where hundreds of thousands have taken shelter. But it has relocated its headquarters to southern Gaza, according to spokesperson Juliette Touma.

Pressed by reporters on whether the army would protect hospitals, UN shelters and other civilian locations, Hagari, the Israeli military spokesperson, warned that “it’s a war zone”. He added: “If Hamas prevents residents from evacuating, the responsibility lies with them.”

The evacuation orders were taken as a further signal of an already expected Israeli ground offensive, though Israel has not yet announced such a decision.

Israeli Prime Minister Benjamin Netanyahu has vowed to “crush” Hamas, which has ruled Gaza since 2007. His government is under intense public pressure to topple the group rather than merely bottle it up in Gaza as it has for years.

A visit by US secretary of state Antony Blinken on Thursday, along with shipments of US weapons, offered a powerful green light for Israel to drive ahead with its retaliation. US defence secretary Lloyd Austin arrived in Israel on Friday.

Still, a ground offensive in densely populated and impoverished Gaza would likely bring even higher casualties on both sides in brutal house-to-house fighting.

Yasser Hassouna, an activist in Gaza City, said “everyone panicked” when they saw the UN staff leaving. “Hamas said it was psychological warfare, and we know there has been a lot of that. No one knows what is real and what is fake news right now.”

Farsakh, of the Palestinian Red Crescent, said there was no way so many people could be safely moved — especially those with ailments. “What will happen to our patients?” she asked. “We have wounded, we have elderly, we have children who are in hospitals.”

Farsakh said many of the medics were refusing to evacuate hospitals and abandon patients. Instead, she said, they called their colleagues to say goodbye.

Beyond the immediate fear and logistical difficulties, the order has deep resonance in Gaza, where more than half of the Palestinians are the descendants of refugees from the 1948 war surrounding Israel’s creation, when hundreds of thousands fled or were expelled from what is now Israel. That exodus is deeply seared into their collective memory.

Already, at least 423,000 people — nearly one in five Gazans — have been forced from their homes by Israeli airstrikes, the UN said on Thursday.

Neighbouring Egypt has meanwhile taken “unprecedented measures” to reinforce its border with Gaza and prevent any breaches, a senior Egyptian security official said.

The official spoke on condition of anonymity because he was not authorised to brief reporters.

Egypt, which made peace with Israel decades ago and has long served as a regional mediator, is staunchly opposed to resettling Palestinians on its territory, both because of the costs involved and because it would undermine their quest for an independent state. The Rafah crossing from Gaza into Egypt, the only one not controlled by Israel, has been closed because of airstrikes.