U.S. President Joe Biden stressed the "critical need" to protect civilians, in a call with Prime Minister Benjamin Netanyahu, who vowed Israel would "continue the war until all of its goals have been achieved" - officials said.
As heavy fighting raged on, the Israeli army said it had struck another 200 targets in the past 24 hours in the besieged Palestinian territory where it is seeking to defeat Hamas and free remaining hostages.
The army said 153 troops had died in Gaza since it launched its ground invasion on October 27. Ten soldiers were lost in battles on Saturday, one of the deadliest days for the Israeli side.
"This is a difficult morning, after a very difficult day of fighting in Gaza," Netanyahu said. "The war is exacting a very heavy price... but we have no choice but to keep fighting."
The army said soldiers had raided a northern Gaza compound near schools, a mosque and a clinic and found "explosive belts adapted for children, dozens of mortar shells, hundreds of grenades and intelligence documents."
Hamas rejected those claims as lies meant "to justify their (Israel) massacring of innocent civilians and their destructive aggression that has affected the entire infrastructure... in the already blockaded Gaza Strip."
The bloodiest ever Gaza war broke out when Hamas attacked southern Israel on October 7 and its Islamist militants killed about 1,140 people, mostly civilians, according to an AFP tally based on Israeli figures.
The gunmen also seized around 250 hostages, 129 of whom are believed to remain in Gaza following the exchange of captives for Palestinians prisoners during a week-long truce late last month.
Israel's withering military campaign, including massive aerial bombardment, has killed 20,424 people, mostly women and children, according to the Hamas-run territory's health ministry.
A picture taken in southern Israel near the border with the Gaza Strip on December 24, 2023, shows Israeli army bulldozers rolling into the Palestinian enclave amid continuing battles between Israel and the militant group Hamas.
Vast areas of Gaza lie in ruins and its 2.4 million people have endured dire shortages of water, food, fuel and medicine due to an Israeli siege, alleviated only by the limited arrival of aid trucks.
Eighty percent of Palestinians in Gaza have been displaced, according to the United Nations, many now shielding against the cold winter in makeshift tents and forced into the south of Gaza even as ground fighting inches closer.
Near the far southern Gaza city of Rafah, Umm Amir Abu al-Awf, 27, suffered wounds to her hand and legs in a strike on her house early Sunday.
"Who won?" she said. "Nothing has been achieved except killing civilians... They keep saying Rafah is safe. It is not safe. Nowhere is safe. Every house has a martyr and injured."
Israel accuses Hamas of using civilians as human shields.
Support among the Israelis for the war has stayed mostly steady despite international calls for ceasefire
Some information in this report came from The Associated Press