Thousands of Palestinians fled the north of the Gaza Strip on Saturday from the path of an expected Israeli ground assault, while Israel pounded the area with more air strikes and said it would 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 4:00 pm.

"Around the Gaza Strip, Israeli reserve soldiers in formation (are) getting ready for the next stage of operations," Israeli military spokesperson 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 during the night, hitting several houses, according to residents who posted appeals on social media platforms.

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. Israel punished us for not wanting to leave our home. Is there brutality worse than this?", a father of three told Reuters by telephone from the hospital, declining to give his name for fear of reprisals.

"I was never going to leave, I prefer to die and not leave, but I can’t see my wife children die before my eyes. We are helpless."

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.

"This is a genocide, not a war, it's genocide. And it's an attempt to displace the people of the Gaza Strip, but this will not happen," said neighbour Mohammad Sadeq. "Martyrs are stuck under the rubble and until now neither us nor the medics nor civil defence were able to take them out."

'This is only the beginning'

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, killed 1,300 people, mainly civilians, and seized 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 1,900 people have been killed, a quarter of them children.

The Gaza Strip is already one of the most crowded areas in the world, and Israel's evacuation order for the northern half meant those fleeing south were forced to shelter with relatives and friends, in schools or in hastily rented apartments.

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.

Gaza authorities said 70 people were killed and 200 were wounded when Israel struck cars and trucks carrying people fleeing the north of the strip for the south on Friday. Reuters could not independently verify that incident.

"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 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.

'Even wars have rules'

The United Nations estimated that 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. 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," the UN office for Coordination of Humanitarian Affairs said in an update.

It said it no longer considered UN premises in the northern half of Gaza to be protected.

The U.N. and other bodies have called on Israel to lift its total siege of Gaza so that 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."