Tens of thousands of people in Gaza have fled their homes after Israel’s military issued a stark evacuation order Friday, according to the United Nation’s humanitarian body, prompting international alarm as Israeli troops gather by the border and as conditions worsen inside the bombarded strip.

Even before the warning, more than 400,000 Palestinians had already been internally displaced by the past week of fighting and relentless Israeli airstrikes, according to a statement by the UN Office for the Coordination of Humanitarian Affairs (OCHA) on Saturday.

But numbers have grown since the Israel Defense Forces (IDF) warned more than 1 million people in northern Gaza to leave their homes immediately, ahead of a possible ground assault in response to the October 7 attacks by Islamist militant group Hamas, CNN reports.

Images from Gaza, which is controlled by Hamas, show a mass rush toward the south of the coastal enclave on Friday. Civilians crammed into cars, taxis, pickup trucks and even donkey-pulled carts; roads were filled with snaking lines of vehicles strapped with suitcases and mattresses.

Those without other options walked, carrying what they could. Some have stayed put regardless, telling CNN they felt nowhere was safe.

Saturday morning marked one week since Hamas’ unprecedented and bloody attack on Israel, which killed more than 1,300 people and led to the capture of civilian and military hostages now believed to be held in Gaza.

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The surprise attack, widely described as Israel’s 9/11, saw waves of heavily armed Hamas fighters rampage through rural Israeli towns, kibbutzim and army bases.

In response, Israel ordered a “complete siege” of Gaza, including blocking food, water and fuel, while mounting its heaviest ever airstrikes on the enclave.

International observers warn the cutoff will see Gaza civilians die by starvation, disease and lack of medical care for the growing numbers of dying and wounded.

At least 1,900 Palestinians have been killed in Gaza from Israeli strikes, according to the Palestinian Ministry of Health. That toll includes 614 children.

Israel has been massing hundreds of thousands of troops and military equipment at the border while it ramps up its siege of Gaza. However, it’s not clear what kind of operation the IDF is preparing for, or when it may take place.