Israeli tanks pushed into southern and eastern districts of the Gazan city of Deir al-Balah for the first time, an area where Israeli sources said the military believes hostages may be held.
The area is packed with Palestinians displaced during more than 21 months of war in Gaza, hundreds of whom fled west or south after Israel issued an evacuation order, saying it sought to destroy infrastructure and capabilities of the militant group Hamas.
Tank shelling in the area hit houses and mosques, killing at least three Palestinians and wounding several others, local medics said.
UN spokesperson Stéphane Dujarric said: "UN staff remain in Deir al-Balah, and two UN guesthouses have been struck, despite parties having been informed of the locations of UN premises, which are inviolable. These locations — as with all civilian sites — must be protected, regardless of evacuation orders."
To the south in Khan Younis, an Israeli airstrike killed at least five people, including a husband and wife and their two children in a tent, medics said.
In its daily update, Gaza's health ministry said at least 130 Palestinians had been killed and more than 1,000 wounded by Israeli gunfire and military strikes across the territory in the past 24 hours, one of the highest such totals in recent weeks.
There was no immediate Israeli comment on the Deir al-Balah and Khan Younis incidents.
Israeli sources have said the reason the army had stayed out of the Deir al-Balah districts was because they suspected Hamas might be holding hostages there. At least 20 of the remaining 50 hostages in captivity in Gaza are believed to be still alive.
Families of the hostages have expressed concern for their relatives and demanded an explanation from Israeli Prime Minister Benjamin Netanyahu, defence minister Israel Katz, and the army chief on how they will protect them.
"The people of Israel will not forgive anyone who knowingly endangered the hostages — both the living and the deceased. No one will be able to claim they didn't know what was at stake," the Hostage Families Forum Headquarters said in a statement.
Gaza health officials have warned of potential "mass deaths" in coming days from hunger, which has killed at least 19 people since Saturday, the territory's health ministry said.
United Nations secretary-general António Guterres was appalled by an accelerating breakdown of humanitarian conditions in Gaza "where the last lifelines keeping people alive are collapsing", Dujarric said.
"He deplores the growing reports of children and adults suffering from malnutrition," he said.
"Israel has the obligation to allow and facilitate by all the means at its disposal the humanitarian relief provided by the United Nations and by other humanitarian organisations."
Health officials say hospitals have been running out of fuel, food aid, and medicine, risking a halt to vital operations.
Gaza health ministry spokesperson Khalil Al-Deqran said medical staff have been depending on one meal a day and that hundreds of people flock to hospitals every day, suffering from fatigue and exhaustion.
In southern Gaza, the health ministry said an Israeli undercover unit had detained Marwan al-Hams, head of Gaza's field hospitals, in a raid that killed a local journalist and wounded another outside a field medical facility run by the International Committee of the Red Cross (ICRC).
An ICRC spokesperson said the ICRC had treated patients injured in the incident but did not comment further on their status. It said it was "very concerned about the safety and security" around the field hospital.
The Israeli military did not immediately respond to a request for comment.
The incursion into Deir al-Balah and growing number of deaths appeared to be complicating efforts to secure a ceasefire between Hamas and Israel in talks mediated by Qatar and Egypt, with the United States' backing.
A Hamas official told Reuters the militant group was angered by the mounting death toll and hunger crisis, and that this could affect the talks on a 60-day truce and hostage deal.
UNRWA, the UN refugee agency dedicated to Palestinians, said on X it was receiving desperate messages from Gaza warning of starvation, including from its own staff, as food prices have soared.
"Meanwhile, just outside Gaza, stockpiled in warehouses, UNRWA has enough food for the entire population for over three months. Lift the siege and let aid in safely and at scale," it said.
The health ministry said on Sunday at least 67 people were killed by Israeli fire as they waited for UN aid trucks to enter Gaza. It said at least 36 aid seekers were killed a day earlier.
Israel's military said its troops had fired warning shots to remove what it said was "an immediate threat". It said initial findings suggested reported casualty figures were inflated.
