The Israeli military said its forces have resumed ground operations in central and southern Gaza, as a second day of airstrikes killed at least 38 Palestinians, according to local health workers.
The renewed ground operations come a day after more than 400 Palestinians were killed in airstrikes in one of the deadliest episodes since the beginning of the conflict in October 2023, shattering a ceasefire that has largely held since January.
The Israeli military said its operations have extended Israel's control over the Netzarim Corridor, which bisects Gaza, and were a "focused" manoeuvre aimed at creating a partial buffer zone between the north and the south of the enclave.
The United Nations said an Israeli airstrike had killed a foreign staffer and wounded five workers at the site of a UN headquarters in central Gaza City. But Israel denied this, saying it had hit a Hamas site, where it had detected preparations for firing into Israeli territory.
Jorge Moreira da Silva, executive director of the UN Office for Project Services, said: "Israel knew that this was a UN premises, that people were living, staying and working there, it is a compound. It is a very well-known place."
UN Secretary-General António Guterres called for a full investigation and condemned all attacks on UN personnel. In a statement he said the strike brought to at least 280 the number of UN colleagues killed in Gaza since 7 October 2023.
In Sofia, the foreign ministry said a Bulgarian working for the UN died in Gaza, citing preliminary information. It was not immediately clear if the Bulgarian was the foreign UN staffer killed in Gaza.
Israel, which has vowed to eradicate Hamas, said its latest onslaught was "just the beginning".
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In the latest violence, local health workers said an Israeli airstrike killed four people and wounded 10 others in a house in northern Gaza town of Beit Hanoun, where the army renewed evacuation orders to residents earlier.
In Beit Lahiya, a new Israeli airstrike killed 14 people at a mourning tent, medics said.
Israel and Hamas accuse each other of breaching the truce, which had offered a respite for Gaza's 2.3 million residents after 17 months of war that has reduced the enclave to rubble and forced most of its population to evacuate multiple times.
Live updates: At least 436 people killed since Israel resume Gaza strikes - civil defence
The Israeli campaign has killed more than 49,000 people in Gaza, Palestinian health authorities say, and caused a humanitarian crisis with shortages of food, fuel and water.
Israel has accused Hamas of using Palestinian civilians as human shields. Hamas denies this and accuses Israel of indiscriminate bombings.
The war - the most devastating episode in decades of Israel-Palestinian conflict - was triggered by a Hamas-led attack on southern Israel on 7 October 2023, in which gunmen killed some 1,200 people and took about 250 hostages, according to Israeli tallies.
Prime Minister Benjamin Netanyahu's decision to resume bombardments has triggered protests in Israel as 59 hostages are still held in Gaza, with 24 of them believed to be still alive.
A coalition of hostage families and protesters against Mr Netanyahu's moves against the judiciary and other parts of the security establishment has regrouped and accuses the prime minister of using the war for political ends.
Palestinian medics said Israeli tank shelling on the main north-south Salahuddin Road killed one Palestinian and wounded others.
Hamas spokesperson Abdel-Latif Al-Qanoua told Reuters the closure of the Salahuddin Road was a "total coup" against the ceasefire agreement and a tightening of the blockade on Gaza.
He renewed his group's interest in concluding the three-phased ceasefire agreement and would welcome any proposal "as long as it is based on launching negotiation on the second phase and a complete end to the war in Gaza".
Earlier, the Israeli army dropped leaflets in northern and southern Gaza, once again ordering residents to evacuate their homes.
Defence Minister Israel Katz issued a video statement warning Gaza residents that evacuation from combat zones would begin shortly.
He said airstrikes were "only the first step" and if the hostages were not released, "Israel will act with force you have not yet seen".
The renewed violence was condemned by Western nations, including France and Germany, as well as Qatar and Egypt, which had been acting as mediators in the ceasefire negotiations.
Israel and Western powers do not want Hamas to play any role in the enclave when the war is over.
Arab nations drew up a plan for peace and reconstruction in Gaza after a proposal from US President Donald Trump to resettle Palestinians and turn it into a "Riviera" of the Middle East triggered outrage in the region. However, the plan has not gained traction.
European Union foreign policy chief Kaja Kallas said she told Israeli Foreign Minister Gideon Saar that the situation in Gaza was "unacceptable".
Jordan's King Abdullah called for the ceasefire to be restored and for aid flows to resume.
"Israel's resumption of attacks on Gaza is an extremely dangerous step that adds further devastation to an already dire humanitarian situation," he said on a visit to Paris for talks with French President Emmanuel Macron.
However, Dorothy Shea, acting US ambassador to the United Nations, said yesterday that the blame for the resumption of hostilities "lies solely with Hamas".
