Prior to the warning, more than 400,000 Palestinians had been internally displaced, the statement added.
Meanwhile, the World Food Programme (WFP) said it had distributed food to 135,000 people in shelters across Gaza on Friday, but warned “humanitarian supplies are running low.”
OCHA added that most people in Gaza now have no access to water.
“As a last resort, people are consuming brackish water from agricultural wells, triggering serious concerns about the spread of waterborne diseases,” it announced.
Israel’s military on Friday ordered 1.1 million people living in northern Gaza to evacuate their homes, amid signs it is set to ramp up its offensive against Hamas following the group’s attacks that killed more than 1,300 people.
For seven days, Israeli warplanes have pounded Gaza with airstrikes that have reduced streets and homes to rubble and killed more than 1,900 people, including 614 children and 370 women, and injured nearly 7,700 others, according to the Palestinian health ministry.
The UN agency for Palestinian refugees, UNRWA, has also announced it will not evacuate its schools where hundreds of thousands have taken shelter. But it relocated its headquarters to southern Gaza, according to spokesperson Juliette Touma.
Philippe Lazzarini, UNRWA’s commissioner general, has warned that “the scale and speed of the unfolding humanitarian crisis is bone-chilling”.
“Gaza is fast becoming a hellhole and is on the brink of collapse,” he stated.
Pressed by reporters on whether the Israeli army would protect hospitals, UN shelters, and other civilian locations, Israeli military spokesperson Daniel Hagari stated the military would keep civilians safe “as much as we can”.
But he warned: “It’s a war zone.”