The war death toll updated daily by the Palestinian health ministry showed an increase of 970 in the space of 48 hours.
Families with young children fled northern Gaza for areas further south, fearing for their lives after Israel urged civilians to leave areas it described as "combat zones".
A Hamas official said the group was open to talks on getting the ceasefire back on track but rejected Israeli demands to renegotiate the three-stage deal agreed with Egyptian, Qatari and US mediators.
"Hamas has not closed the door on negotiations but we insist there is no need for new agreements," Taher al-Nunu told AFP.
"We have no conditions, but we demand that the occupation be compelled to immediately halt its aggression and war of extermination, and begin the second phase of negotiations."
Negotiations have stalled over how to proceed with a ceasefire whose first phase expired in early March, with Israel and Hamas disagreeing on whether to move to a new phase intended to bring the war to an end.
Israel and the United States have sought to change the terms of the deal by extending stage one.
That would delay the start of phase two, which was meant to establish a lasting ceasefire and an Israeli withdrawal from Gaza, and was swiftly rejected by Hamas, which demanded full implementation of the original deal.
"There is no need for new agreements in light of the existing agreement signed by all parties," Nunu said.
'Only the beginning'
Israel and the United States have portrayed Hamas's rejection of an extended stage one as a refusal to release more Israeli captives in exchange for Palestinian prisoners.
Netanyahu's office said he ordered the renewed strikes on Gaza after "Hamas's repeated refusal to release our captives".
In a televised address late Tuesday, the premier said: "Hamas has already felt the strength of our arm in the past 24 hours. And I want to promise you, and them, this is only the beginning."
The White House said Israel consulted US President Donald Trump's administration before launching the strikes.
The intense Israeli bombardment sent a stream of new casualties to the few hospitals still functioning in Gaza and triggered fears of a return to full-blown war after two months of relative calm.
Two people, including a United Nations employee, were killed when a UN building in Deir el-Balah, in the central Gaza Strip, was hit, according to a UN source.
One of those killed was employed by the UN Office for Project Services (UNOPS), the source told AFP.
According to a UN statement, "an explosive ordnance was dropped or fired at the infrastructure and detonated inside the building.
"We don't know at this stage what type it was (airdrop weapons, artillery, rocket)," the statement said.
The Palestinian health ministry blamed Israel, while the Israeli military denied it had struck the UN compound in Deir el-Balah.
AFPTV footage showed UN vehicles and an ambulance transporting three men to Al-Aqsa Martyrs Hospital.
Two of them appeared to have leg injuries and a third had bandages on both arms and abdomen, with traces of blood on his chest.
Thousands of Israelis massed in Jerusalem on Wednesday, accusing Netanyahu of resuming strikes on Gaza without regard for the safety of the remaining captives.
"Many people here in Israel are so frustrated with the operation that began yesterday because it's obvious it will not... make Hamas more flexible and bring the release of hostages," said Palestinian affairs expert Michael Milshtein of Tel Aviv University's Moshe Dayan Center.
'Shattering' hopes
Governments in the Middle East, Europe and beyond called for the renewed hostilities to end.
German Foreign Minister Annalena Baerbock said Israel's raids on Gaza "are shattering the tangible hopes of so many Israelis and Palestinians of an end to suffering on all sides".
European Union foreign policy chief Kaja Kallas said she told her Israeli counterpart Gideon Saar that the new strikes on Gaza were "unacceptable".
Both Egypt and Qatar, which brokered the Gaza ceasefire alongside the United States, condemned Israel's resort to military action.
Israel's resumption of military operations in Gaza, after it already halted all humanitarian aid deliveries to Gaza this month, drew an immediate political dividend for Netanyahu.
The far-right Otzma Yehudit party, which quit his ruling coalition in January in protest at the Gaza ceasefire, rejoined its ranks with its firebrand leader Itamar Ben Gvir again becoming national security minister.
Since Oct.7, 2023, the Israel's genocidal war on the Gaza Strip has killed at least 49,547 people, most of them women and children, with more than 112,500 others injured, according to the Palestinian health ministry's latest figures.
* This story was edited by Ahram Online.
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