Thousands of Palestinians are fleeing the north of the Gaza Strip from the path of an expected Israeli ground assault, while Israel pounds the area with more air strikes and says it will keep two roads open to let people escape.
Israel had given the entire population of the northern half of the Gaza Strip, which includes the enclave's biggest settlement Gaza City, until Saturday morning to move south.
It announced overnight that it would guarantee the safety of Palestinians fleeing the area on two main roads until 4pm on Saturday.
"Around the Gaza Strip, Israeli reserve soldiers in formation (are) getting ready for the next stage of operations," Israeli military spokesman Lieutenant Colonel Jonathan Conricus told a video briefing early on Saturday.
"They are all around the Gaza Strip, in the south, in the centre and in the north, and they are preparing themselves for whatever target they get, whatever task."
In Gaza City's Tel Al-Hawa neighbourhood, part of the area Israel has ordered evacuated, warplanes bombed a residential area overnight, hitting several houses, according to residents who posted appeals on social media.
Hundreds of residents of the area took refuge at the nearby Quds hospital and planned to join those fleeing to the south in the morning.
"We lived a night of horror," a father of three told Reuters by telephone from the hospital, declining to give his name for fear of reprisals.
"Israel punished us for not wanting to leave our home.
"Is there brutality worse than this?"
In Khan Younis, in the southern Gaza Strip, Israeli planes struck a four-storey building, killing and wounding several people.
Dozens of Palestinians were rushing there to help rescue people trapped in the rubble.
Israel has vowed to annihilate the Hamas militant group that controls Gaza in retaliation for a rampage by the group's fighters, who stormed through Israeli towns a week ago, killing 1300 people, mainly civilians, and seizing scores of hostages.
Israel has since put the Hamas-run Gaza Strip, home to 2.3 million Palestinians, under a total siege and bombarded it with unprecedented air strikes.
Gaza authorities say more than 1900 people have been killed, a quarter of them children.
Israel says the order to leave is temporary, and a humanitarian gesture to protect residents from harm while it roots out Hamas fighters entrenched in Gaza City.
The United Nations says so many people cannot be safely moved inside the besieged enclave without causing a humanitarian disaster.
Hamas has vowed to fight until the last drop of blood, and says the order to leave is a trick to force residents to give up their homes.
Gaza City mosques have blared calls telling people to stay.
"We are striking our enemies with unprecedented might," Israeli Prime Minister Benjamin Netanyahu said in a rare statement televised after the Jewish Sabbath began on Friday.
"I emphasise that this is only the beginning."
Conricus, the military spokesman, said: "The end state of this war is that we will dismantle Hamas and its military capability and fundamentally change the situation so that Hamas never again has the ability to inflict any damage on Israeli civilians or soldiers."
The Israeli military said on Friday that tank-backed troops had mounted raids to hit Palestinian rocket crews and gather information on the location of hostages, the first official account of ground troops in Gaza since the crisis began.
The UN estimated tens of thousands of Palestinians headed south from northern Gaza after the Israeli order on Friday, adding to 400,000 Gazans already displaced earlier in the week.
"No safe corridors were initially provided for people to safely comply with the orders to move southwards," the UN office for Co-ordination of Humanitarian Affairs said in an update.
"Hundreds of people, including families, had to flee on foot.
"There are concerns about the food security, and access to water, shelter, and health care of the new IDPs."
It said it no longer considered UN premises in the northern half of Gaza to be protected.
The UN and other bodies have called on Israel to lift its total siege of Gaza so aid can get in.
"We need immediate humanitarian access throughout Gaza, so that we can get fuel, food and water to everyone in need," UN Secretary-General Antonio Guterres said on Friday.
"Even wars have rules."
The United States has firmly backed its ally Israel but has called on it to avoid civilian casualties.
President Joe Biden said tackling the humanitarian crisis was a top priority.
US teams in the region were working with Israel, Egypt, Jordan, other Arab governments and the United Nations, he said.
"The overwhelming majority of Palestinians had nothing to do with Hamas and Hamas' appalling attacks," Biden said in a speech.
"And they're suffering as a result as well."
