Israel unleashed hundreds of air strikes on Gaza as the war with Hamas restarted on Friday within minutes of a week-long truce ending.
More than 200 bombs and missiles smashed the enclave as Israel widened its deadly onslaught on Hamas.
Missiles hit southern Gaza, including the community of Abassan, east of the city of Khan Yunis.
Both sides blamed each other for the ending of the truce. Israel's leaders said that Hamas violated it, while the militant Palestinian group blamed Israel for refusing offers to free more hostages.
The Israeli military said on Friday that it had hit "over 200 terror targets".
"Over the last few hours, ground, air and naval forces struck terror targets in the north and south of the Gaza Strip, including in Khan Yunis and Rafah," it said in a statement.Many of Israel's attacks on Saturday were focused on the Khan Younis area in southern Gaza, where the military said it had struck more than 50 Hamas targets with airstrikes, tank fire and its navy.
More than 1.7 million of Gaza's population of 2.3 million are now based in the south, according to the Red Cross. Aid groups have reported a humanitarian crisis with shortages of food, water and medicine, and a rise in communicable diseases.
The US Secretary of State, Antony Blinken, urged Israel to take steps to limit harm to civilians, saying he "underscored the imperative for the US that the massive loss of civilian life and displacement of the scale that we saw in northern Gaza not be repeated in the south"
The IDF dropped leaflets over Khan Younis urging residents to flee to a designated "safe zone", and published a map offering guidance on evacuation routes for civilians.
But the only safe zone announced so far is the coastal town of Al-Mawasi, which aid groups say is too small to accommodate hundreds of thousands of refugees at just five square miles.