JERUSALEM >> Israel’s military told some 1 million Palestinians living in Gaza on Friday to evacuate the north, according to the United Nations, an unprecedented order for almost half the population of the sealed-off territory ahead of an expected ground invasion against the ruling Hamas militant group.

The U.N. warned that so many people fleeing en masse would be calamitous, and Hamas, which staged a shocking and brutal attack on Israel this week, dismissed it as a ploy and called on people to stay in their homes, adding to widespread panic.

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

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“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 spokeswoman for the Palestinian Red Crescent in Gaza City, breaking into heaving sobs.

The week-old war has already claimed over 2,800 lives on both sides and sent tensions soaring across the region. Weekly Muslim prayers later Friday could bring mass protests at a flashpoint holy site in east Jerusalem, the occupied West Bank and elsewhere. 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.

Israel has bombarded Gaza non-stop since a weekend attack in which Hamas fighters stormed into the country’s south and massacred hundreds of people, 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 has killed 13 of the hostages, including foreigners. It did not give the nationality of the foreigners, saying they were killed over the last 24 hours. There was no immediate Israeli comment.

The Israeli military sent one evacuation order directly Friday morning, warning the residents of Gaza City to flee south in the narrow coastal territory, which is just 40 kilometers (25 miles) long. Israel said it needed to target Hamas’ military infrastructure, much of which is buried deep underground.

Israeli military spokesman Jonathan Conricus said the military will use “significant force” while making “extensive efforts to avoid harming civilians.”

The United Nations said it received a separate directive from the military, giving all 1.1 million civilians of northern Gaza 24 hours to flee south. The Israeli military did not immediately confirm the broader evacuation order.

“The United Nations considers it impossible for such a movement to take place without devastating humanitarian consequences,” U.N. spokesperson 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.”

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 to punishing Israeli strikes.

The flurry of directives was taken as a further signal of an already expected Israeli ground offensive, though Israel has not yet announced such a decision. On Thursday, the military said it was prepared for a ground operation if one was ordered.

A ground offensive in densely populated and impoverished Gaza would likely bring even higher casualties on both sides in brutal house-to-house fighting — but the Israeli government is under intense public pressure to topple Hamas.

The group, which has ruled Gaza since 2007, called on Palestinians to stay in their homes, saying “the (Israeli) occupation 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.”

“This is chaos, no one understands what to do,” said Inas Hamdan, an officer at the U.N. agency for Palestinian refugees in Gaza City, while she grabbed whatever she could throw into her bags as the panicked shouts of her relatives could be heard around her. She said all the U.N. staff in Gaza City and northern Gaza had been told to evacuate south to Rafah.

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 U.N. said Thursday.

The U.N. said the broad evacuation warning it received for all of Gaza’s north also applies to all U.N. staff and to the hundreds of thousands who have taken shelter in U.N. schools and other facilities since Israel launched round-the-clock airstrikes Saturday.

Another U.N. official said that the United Nation is trying to get clarity from Israeli officials at the senior-most political level.

Neighboring Egypt has meanwhile taken “unprecedented measures” to prevent a breach of its border with Gaza, a senior Egyptian security official said. He said a potential ground invasion would be a “grave mistake.”

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 ben closed because of airstrikes.

Israeli Prime Minister Benjamin Netanyahu vowed to “crush” Hamas. A visit by U.S. Secretary of State Antony Blinken on Thursday, along with shipments of U.S. weapons, offered a powerful green light for Israel to drive ahead with its retaliation.

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 — and the ensuing Israeli bombardment has killed more than 1,530 people in Gaza, according to authorities on both sides. Israel says roughly 1,500 Hamas militants were killed inside Israel, and that hundreds of the dead in Gaza are Hamas members. Thousands have been wounded on both sides.

Amid concerns that the fighting could spread in the region, Syrian state media reported that Israeli airstrikes on Thursday put two Syrian international airports out of service.

On Thursday, Israel said its complete siege of the territory — which has left Palestinians desperate for food, fuel and medicine — would remain in place until Hamas militants free some 150 hostages taken during their grisly weekend incursion.

“Not a single electricity switch will be flipped on, not a single faucet will be turned on and not a single fuel truck will enter until the Israeli hostages are returned home,” Israeli Energy Minister Israel Katz said on social media.

Shurafa reported from Gaza City, Gaza Strip. Associated Press writers Amy Teibel and Isabel DeBre in Jerusalem; Sam McNeil in Be’eri, Israel; Jack Jeffrey and Samy Magdy in Cairo; Samya Kullab in Baghdad and Kareem Chehayeb in Beirut contributed to this report.