Israel conducted this week the deadliest wave of air strikes since the start of a truce in January, killing hundreds of people, according to the health ministry in Gaza.
"Over the past 24 hours, IDF (the Israeli army) forces have begun targeted ground operations in the central and southern Gaza Strip to expand the security perimeter and create a partial buffer between the north and south of the Strip," the army said in a statement.
"Residents of Gaza, this is the last warning," Defence Minister Israel Katz said.
"Take the advice of the President of the United States. Return the hostages and remove Hamas, and other options will open up for you -- including the possibility of leaving for other places in the world for those who want to."
He was referring to a warning earlier this month by US President Donald Trump, who said: "To the People of Gaza: A beautiful Future awaits, but not if you hold Hostages. If you do, you are DEAD!"
Long lines of fleeing civilians filled the roads of Gaza on Wednesday as Israel kept up its renewed bombardment despite a chorus of calls from foreign governments to preserve the ceasefire.
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".
Impasse
So far, Hamas has not responded militarily to the strikes, and an official from the group said it was open to talks on getting the ceasefire back on track.
He rejected, however, 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 stalled over how to proceed with the 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 -- a stance swiftly rejected by Hamas, which demanded full implementation of the original deal.
'So frustrated'
Israel and the United States have portrayed Hamas's rejection of an extended phase 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 hostages".
The White House said Israel consulted with it 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.
A UN Office for Project Services employee was killed and at least five other people were wounded when a UN building in the central city of Deir el-Balah was hit, the agency said.
A UN statement said "An explosive ordnance was dropped or fired at the infrastructure and detonated inside the building", adding the type of ordnance was so far unknown.
The health ministry in Gaza 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 violence 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".
Israel's resumption of military operations, 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 its ruling coalition in January in protest at the Gaza ceasefire, rejoined its ranks.
The war began on October 7, 2023, and resulted in 1,218 deaths according to Israeli figures.
Of the 251 captives seized during October 7, 2023, 58 are still in Gaza, including 34 the Israeli military says are dead.
The Israeli war in Gaza killed more than 48,570 Palestinians, mostly women and children.
Gaza's civil defence agency said Wednesday that at least 436 people had been killed since Israel resumed intense strikes on the Palestinian territory.
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