Israel is at war with Hamas. Here's what to know

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The Israeli military on Friday called on all civilians in Gaza City to evacuate "southwards" as it maintains its bombardment of the coastal enclave in response to last weekend's Hamas attacks that killed more than 1,300 people.

That would involve moving more than 1.1 million people from the north to the south of the besieged enclave in 24 hours, the United Nations said. The task would be "impossible" without causing major humanitarian consequences, it warned.

Israeli airstrikes have been pounding Gaza since the day of the attack, leveling entire neighborhoods, including schools and mosques. Israel says it strikes Hamas targets and that the group has used civilians as human shields.

Israel's assault has killed at least 1,900 people in Gaza, including hundreds of children. At least 423,000 people have been displaced, the UN has said.

Here's what we know so far.

Militants from Gaza fired thousands of rockets towards Israeli towns on Saturday morning, before breaking through the heavily fortified border fence with Israel and sending militants deep into Israeli territory. There, Hamas gunmen killed hundreds of people, including civilians and soldiers, and took up to 150 hostages, according to Israeli authorities. They said the bodies of some 1,500 Hamas fighters had been recovered inside Israel since Saturday's assault.