JERUSALEM — 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 U.N. warned that so many people fleeing en masse — with just a 24-hour deadline — 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 across the Middle East, 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 barred 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 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 airstrikes killed 13 of the hostages in the past day. It said the dead included foreigners but did not give their nationalities.

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

Israel said Thursday it would allow no supplies into Gaza until Hamas frees the hostages.

The military urged civilians in Gaza’s north to move south — an order that the U.N. said affects 1.1 million people. If carried out, that would mean the territory’s entire population cramming into roughly the southern half of the strip, which is only 25 miles long, even as Israeli strikes continued Friday to hammer neighborhoods across southern Gaza.

Israel said it needed to target Hamas’ military infrastructure, much of which is buried deep underground. Another spokesperson, Jonathan Conricus, said the military would take “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.

“The camouflage of the terrorists is the civil population,” Israeli Defense Minister Yoav Gallant said at a news conference with U.S. Defense Secretary Lloyd Austin. “Therefore, we need to separate them. So those who want to save their life, please go south.”

UN Warns of ‘Devastating Humanitarian Consequences’

But U.N. 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 urged 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 — already struggling with high numbers of dead and injured — and that hospital staff would not heed the warning.

“We cannot evacuate hospitals and leave the wounded and sick to die,” spokesperson Ashraf al-Qidra said.

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

Pressed by reporters on whether the army would protect hospitals, U.N. 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.”

Clive Baldwin a senior legal adviser at the New York-based Human Rights Watch, said “ordering a million people in Gaza to evacuate, when there’s no safe place to go, is not an effective warning.”

“The roads are rubble, fuel is scarce, and the main hospital is in the evacuation zone,” he said. “World leaders should speak up now before it is too late.”

The evacuation order was taken as a further signal of an already expected Israeli ground offensive, though no such decision has been announced.

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 U.S. Secretary of State Antony Blinken on Thursday, along with shipments of weapons, offered a powerful green light for Israel to drive ahead with its retaliation. Defense Secretary Austin, who met with Israeli leaders Friday, reiterated the United States’ ironclad support for Israel, saying military assistance would flow in “at the speed of war.”

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.

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.”

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 U.N. said Thursday.

Neighboring 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 authorized 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.

Hamas’ unprecedented assault last Saturday, and days of heavy rocket fire since, have killed more than 1,300 people in Israel, including 247 soldiers — a toll unseen in Israel for decades. The ensuing Israeli bombardment has killed more than 1,530 people in Gaza, according to the Palestinian Health Ministry.

Israel says roughly 1,500 Hamas militants were killed inside Israel, and that hundreds of the dead in Gaza are Hamas members.