An Israeli strike flattened a high-rise in Gaza City on Saturday -- the second in as many days -- ahead of a planned offensive to capture the area.

Israel has been warning for weeks of a new assault on the territory's largest urban centre, without issuing a timeline.

Israeli army spokesman Nadav Shoshani said on Thursday that the start of the ground campaign to take Gaza City would not be announced to "maintain the element of surprise."

It has stepped up air strikes in the area and operations on the city's outskirts despite calls to abandon the plan, which has sparked widespread fears it could worsen already-dire humanitarian conditions.

Last month, the United Nations officially declared famine in Gaza after a 23-month Israeli blockade on the entry of humanitarian aid to the strip.

On Saturday, the military announced it struck a Gaza City high-rise.

Witnesses identified the building as the Sussi residential tower and said it was destroyed, with video shared by Israeli Defence Minister Israel Katz showing the roughly 15-storey structure buckling to the ground in a cloud of dust and smoke.

"We're continuing," Katz said in the post, after having shared a video the previous day of another Gaza City high-rise being destroyed.

Israeli warplanes destroyed Mushtaha Tower, a high-rise in Gaza City, on Friday, giving residents only 15 minutes to evacuate. 

The military has said that in the coming days it will continue to target structures in the area, particularly tall buildings.

It also issued an evacuation order for another high-rise on Saturday, warning of an imminent strike and telling people to leave to the south.

A military spokesperson had earlier called on residents to leave the city for Al-Mawasi, along the southern coast, where the army said humanitarian aid and medical care would be provided.

Israel first declared Al-Mawasi a safe zone early in the war, but has carried out repeated strikes on it since.

Gaza City residents said they believed it made little difference whether they stayed or fled.

"Some say we should evacuate, others say we should stay," said Abdel Nasser Mushtaha, 48, a resident of the city's Zeitun neighbourhood now sheltering in a tent in the Rimal area.

"But everywhere in Gaza there are bombings and deaths. For the past year-and-a-half, the worst bombings that caused massacres of civilians have been in Al-Mawasi, this so-called humanitarian zone," he added.

"It no longer makes any difference to us," said his daughter Samia Mushtaha, 20. "Wherever we go, death pursues us, whether by bombing or hunger."

Israel has faced mounting domestic and international pressure to end the nearly two-year genocidal war on Gaza.

Netanyahu has refused to respond to a 60-day truce/prisoner swap proposal, mediated by Egypt and Qatar, that was previously agreed to by Israel and accepted by Hamas on 18 August.

At the White House on Friday, President Donald Trump said the United States was in talks with Hamas over the captives being held in Gaza.

"We're in very deep negotiation with Hamas," said Trump.

The UN estimates nearly one million people remain in and around Gaza City, where it declared a famine last month. It has warned of a looming "disaster" if the assault proceeds.

The vast majority of Gaza's population of more than two million people have been displaced more than once during the war.

Israel's genocidal war on Gaza has killed at least 64,368 Palestinians, most of them women and children.