An Israeli strike flattened a high-rise in Gaza City on Saturday -- the second in as many days -- after the military warned people to flee south to a "humanitarian zone" 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.

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.

On Saturday, the military announced it struck a Gaza City high-rise, saying "Hamas terrorists installed intelligence gathering equipment and positioned observation posts in the building in order to monitor" Israeli troops, adding it had taken "measures to mitigate harm to civilians".

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........

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