The air attacks and artillery shelling come minutes after the military ordered the evacuation that Palestinian Civil Defence in Gaza says affects more than 400,000 people.
The victims were killed by tank salvoes in the town of Bani Suheila and other towns just east of the southern city of Khan Younis, with the area also bombarded by air, according to medics.
The Health Ministry in Gaza said the dead included several women and children and that dozens of other people had been injured by Israeli fire.
Some 400,000 people are living in the targeted areas and dozens of families have begun to leave their houses, Palestinian officials said, adding that they were not given time to evacuate before the Israeli strikes began.
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At Nasser Hospital, some people stood outside the morgue to bid farewell to dead relatives before burials.
“We are tired, we are tired in Gaza, every day our children are martyred, every day, every moment. This is the blood of our children, it has not yet dried,” said Ahmed Sammour, who lost several relatives in the Israeli bombing.
“No one told us to evacuate. They brought four floors crashing down on civilians … and the bodies they could reach, they brought to the refrigerator [morgue],” Sammour told Reuters.
The office says Israeli attacks on Khan Younis governorate also injured 172 people within 10 hours.
In a statement, the office said it received more than 1,200 appeals for help from besieged families asking to be rescued.
“This morning, the Israeli occupation army began a new aggression against Khan Younis Governorate committing crimes against humanity through indiscriminate bombing of citizens’ homes and groups of displaced people in the area,” the office said.
Of the 57 people killed, “49 were taken to Nasser Medical Complex, while other martyrs were taken to other hospitals,” it added.
The army says in a statement posted on X that the officer was killed by a grenade explosion this morning in the Gaza Strip.
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The army specified that this incident did not occur during “operational activity” in Gaza, saying an investigation into its circumstances is ongoing.
The Israeli prime minister is travelling to the US, where he is due to meet Biden and address a joint session of Congress this week.
His visit to Washington, DC, is expected to be met with protests by thousands of people opposing Israel’s war on Gaza and US support for Israel.
Hamas says the attack, which has killed and wounded dozens of people and displaced many families, will not deter Palestinians from remaining “steadfast in their land”.
“We call upon the international community and the United Nations to urgently intervene to stop the systematic Zionist killing of our people, who are facing a genocide,” Hamas said in a statement on its Telegram channel.
“We affirm that our people will remain steadfast, and the resistance will remain committed to confronting this criminal enemy that seeks to expel our people and obliterate our national cause.
“Gaza will remain unbreakable until the occupation is defeated on the path to freedom, return, and self-determination,” it said.
