The Israeli military warned Palestinians in Gaza City to leave for the south on Saturday before bombing a high-rise tower as its forces advanced deeper into the enclave's largest urban area.

Israeli forces have been carrying out an offensive on the suburbs of the northern city for weeks after Israeli Prime Minister Benjamin Netanyahu ordered the military to capture it.

Netanyahu says Gaza City is a Hamas stronghold and capturing it is necessary to defeat the Palestinian Islamist militants, whose October 2023 attack on Israel sparked the war.

The assault threatens to displace hundreds of thousands of Palestinians sheltering there from nearly two years of fighting.

Before the war, around a million people, nearly half of Gaza's population, lived in the city.

Israeli military spokesperson Avichay Adraee wrote on X that residents should leave the city for a designated coastal area of Khan Younis in southern Gaza, assuring those fleeing that they would be able to receive food, medical care and shelter there.

The designated area was a "humanitarian zone", Adraee said.

The military also issued so-called "evacuation warnings" to civilians in certain areas of the city, warning it was about to carry out attacks.

The military later bombed a high-rise Gaza City tower that it said was being used by Hamas, without providing evidence to support the assertion.

It said civilians were warned in advance.

Israel's Defence Minister Israel Katz shared a video on X of what appeared to be the multi-storey building collapsing after the strike, sending a cloud of dust and debris into the air.

It was not immediately clear if there were any casualties.

The Israeli military said Hamas used the building to gather intelligence and that explosive devices had been planted nearby.

Hamas denied using the building for military purposes, and Palestinians said it had been used to shelter the displaced.

"These towers are strictly monitored, entry is permitted exclusively for civilians," Hamas said in a statement, adding the Israeli allegations constitute "a systematic forced displacement" plan.

Gaza health authorities said Israeli strikes and gunfire killed at least 40 people across the Gaza enclave on Saturday, at least half of them in Gaza City. (Reuters)