Residents said the bombardment from air, land and sea was the most intense in southern Gaza since the war began in October.
Troops moved for the first time into Al-Mawasi district near the Mediterranean coast, west of Khan Younis, the main city in southern Gaza. There, they stormed Al-Khair hospital and arrested medical staff, Gaza health ministry spokesman Ashraf al Qidra told Reuters.
There was no word from Israel on the situation at the hospital, and the military spokesperson's office had no comment. The military said later that three Israeli soldiers were killed on Monday in southern Gaza.
Qidra said at least 50 people were killed on Sunday night in Khan Younis, while the sieges at medical facilities meant dozens of dead and wounded were beyond the reach of rescuers.
White House National Security Council spokesman John Kirby on Monday said Israel had a right to defend itself but added: "We expect them to do so in accordance with international law and to protect innocent people in hospitals, medical staff and patients as well, as much as possible."
Israel says Hamas fighters operate in and around hospitals, which Hamas and medical staff deny.
Israel launched an offensive last week to capture Khan Younis, which it now says is the principal headquarters of the Hamas responsible for the Oct. 7 attacks on southern Israel that killed 1,200 people, according to Israeli tallies.
Israel has vowed to wipe out Hamas, the Palestinian Islamist movement that rules Gaza and is sworn to Israel's destruction.
At least 25,295 Gazans have been killed since Oct. 7, Gaza health authorities said in an update on Monday.
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International concern has mounted over the Palestinian death toll from Israel's assault on the densely populated enclave and a humanitarian crisis afflicting hundreds of thousands of people.
The United States has urged Israel to reduce civilian harm in Gaza, called on it to stop violence by Israeli settlers against Palestinians in the occupied West Bank and has said it still believes a two-state solution to the Israeli-Palestinian conflict is possible.
In Brussels on Monday, Israeli Foreign Minister Israel Katz sidestepped discussion of a two-state solution at a meeting with European Union counterparts, opting to show them aspirational videos of future infrastructure projects.
Israel has pushed back against calls for a Palestinian state, and Prime Minister Benjamin Netanyahu on Sunday reaffirmed a hard line, saying any Palestinian state would pose "an existential danger" to Israel.
EU foreign policy chief Josep Borrell has circulated a discussion paper with a roadmap to peace that calls for a preparatory peace conference organised by the EU and Arab nations. The U.S. and U.N. were also invited to be conveners.
Washington on Monday called for an investigation and accountability "as appropriate" following the West Bank death on Friday of a 17-year-old Palestinian American, who Palestinian health officials say was killed by Israeli security forces.
The Israel-occupied West Bank has seen a surge of violence in parallel to the Gaza war that erupted in October.
The American-born youth's uncle told Reuters he died during clashes with the Israeli military that included stone throwing by Palestinians.
