An Israeli airstrike that hit several houses and a multi-storey residential building in the northern Gaza town of Beit Lahiya has killed at least 73 people, according to medics and Hamas-run media outlets.
No official casualty figures were immediately available from the Gaza health ministry, however, a senior health ministry official said the figures were accurate.
The Israeli military said it was looking into the incident but said the numbers published by Hamas-affiliated organisations were "exaggerated".
It added the figures did not align with its own information, the precise munitions used or the accuracy of the strike, which it said was directed at a Hamas target.
"This is a war of genocide and ethnic cleansing," the Hamas media office said. "The occupation has conducted a horrifying massacre in Beit Lahiya."
Health officials said ongoing rescue operations Beit Lahiya were being hampered by the cut-off of telecommunication and internet services for a second day.
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Earlier on Saturday, at least 35 people, including children, were killed in strikes across Gaza, according to the Gaza health ministry.
This includes a strike on the Maghazi refugee camp that killed 11 members of the same family, and another 10 who died when a house in the town of Zawayda was hit, according to the al Aqsa Martyrs Hospital in Deir al Balah.
It comes after a reported 33 people were killed in a strike on the Jabalia refugee camp late on Friday night, according to the Palestinian WAFA news agency.
More than 42,000 Palestinians have been killed and almost 100,000 others wounded during Israel's offensive in Gaza since 7 October 2023, according to Gaza's Hamas-run health ministry.
The health ministry does not differentiate between civilians and militants in its reporting of the figures.
Shockwaves from the killing of Hamas leader Yahya Sinwar continue to reverberate in the region, and speculation persists Israel could strike Iran's nuclear facilities.
Drone launched at Netanyahu's house
In Israel, a drone was launched towards the house of Israel's Prime Minister Benjamin Netanyahu, his spokesman said.
Prime Minister Sir Keir Starmer said he was "alarmed to hear" about the incident in a call to Mr Netanyahu on Saturday afternoon.
Sir Keir described Sinwar as a "brutal terrorist" and said the "world is a better place without him" as he discussed the situation in the Middle East with Mr Netanyahu.
"He also discussed with Prime Minister Netanyahu the opportunity presented by Sinwar's death to halt the fighting and get the hostages out," a spokesperson said.
"The prime minister also stressed the importance of getting much more aid into Gaza."
Sirens warned of incoming fire from Lebanon, with a drone launched towards a house owned by Prime Minister Netanyahu in Caesarea, the Israeli government said.
Neither Mr Netanyahu nor his wife were home at the time and there were no casualties, according to the Israeli leader's spokesperson.
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In a statement on X, Mr Netanyahu said: "The agents of Iran who tried to assassinate me and my wife today made a bitter mistake.
"This will not deter me and the State of Israel from continuing the war of revival against our enemies to ensure our security for generations.
"Anyone who harms the citizens of the State of Israel will pay a heavy price for it."
The IDF said 55 projectiles were fired at northern Israel from Lebanon on Saturday morning, with some intercepted on route.
Deputy commander of Hezbollah killed
Israel also said on Saturday it killed Nasser Rashid in the town of Bint Jbeil in southern Lebanon.
The deputy commander of Hezbollah supervised attacks against Israel, the army said.
Lebanon's health ministry said an airstrike on Saturday hit a vehicle on a main highway north of Beirut, killing two people. It was unclear who was in the car.
Israel's conflict with the militant group has intensified in recent weeks as fallout from the killing of leader Hassan Nasrallah continues.
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Khamenei: Hamas will live on
Meanwhile, Iran's Supreme Leader Ayatollah Ali Khamenei says the death of Hamas leader Sinwar will not halt the so-called "Axis of Resistance" and that Hamas would live on.
"His loss is undoubtedly painful for the Axis of Resistance, but this front did not cease advancing with the martyrdom of prominent figures," he said in a statement.
"Hamas is alive and will remain alive."
Read more:
Hamas sets out conditions for return of Israeli hostages
What Sinwar's death might mean for war
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Leaflets dropped over southern Gaza
Israeli planes dropped leaflets over southern Gaza on Saturday, showing a picture of the recently assassinated Sinwar.
They carried the message: "Hamas will no longer rule Gaza", echoing language used elsewhere by Israel's most senior politicians.
Meanwhile, two people were killed as they travelled along one of Lebanon's main roads near the Christian-majority town of Jounieh in the first such attack on the area.
A spokesperson for Israel's military said it was looking into it.
Another strike killed at least four people in Lebanon's Bekaa Valley, health authorities said.
In London, several thousand people turned out for another demonstration against the war in the Middle East.