Warning that an attack of "great force" would soon be underway, the Israel Defense Forces told the entire population of Gaza City to move to southern Gaza.

Israel on Tuesday ordered up to 1 million residents of Gaza City, including displaced people sheltering there, to evacuate immediately ahead of an expected military operation involving "great force." Photo by Omar Ishtiwi/UPI UPI

Sept. 9 (UPI) -- The Israeli military on Tuesday ordered around 1 million residents of Gaza City to immediately evacuate to the south, warning an attack of "great force" was imminent.

Using the hashtag "Urgent," the Israel Defense Forces told residents of Gaza's most populous city via an online post that, for their own safety, they should "move immediately toward the humanitarian zone in Al-Mawasi."

"To all residents of the city of Gaza and those present in all its neighborhoods, from the Old City and Tafah to the east and up to the sea to the west, The Defense Army is determined to eliminate Hamas and will operate in the Gaza city area with great force, as it has operated in various parts of the Strip. Staying in the city is extremely dangerous," the message reads.

Israel said that people who do as it asks will be provided with food and medical care on arrival at Al-Mawasi, a designated safe zone, near Khan Younis.

However, the United Nations warned there was nowhere in middle or southern Gaza that was "safe" because even so-called humanitarian sectors had previously come under heavy bombardment and that facilities at Al-Mawasi were already overstretched due to overcrowding.

The head of the U.N. Human Rights Office for the Palestinian territories, Ajith Sunghay, told the BBC that the Israeli assault on Gaza City would be "catastrophic."

"What we are going to see is something we haven't seen before. They're destroying everything that is needed for civilian survival. All civilian structures -- whether it is water tanks, roads, buildings, they've all been destroyed -- we have seen in the northern part of Gaza they have levelled it, and we fear Gaza City will face the same consequences."

Sunghay said many thousands of people would heed the calls to head southwards, but a significant number would not do so because they were aware that they would never be able to come back.

The evacuation order came as Defense Minister Israel Katz threatened to raze all of Gaza and the military stepped up its targeting of high-rise buildings in Gaza City.

Katz claimed in a post on X that 30 multi-story "terror buildings" were attacked and destroyed on Monday alone to clear the path for Israeli ground forces. The post was accompanied by footage of a high-rise building being brought down, apparently by explosive charges placed in and around the lower floors.

"The hurricane storm struck Gaza yesterday with unprecedented force. If Hamas terrorists do not lay down their arms and release all the hostages, they will be destroyed and Gaza will be ruined," wrote Katz.