Israeli strikes have killed at least 73 people, including women and children, in the city of Beit Lahia in northern Gaza, the strip's Hamas-run authorities say.
Dozens of others are injured and many are still trapped under the rubble after the bombing late on Saturday night, officials added, reports the BBC.
Israel said it was checking reports of casualties but said the figures published by Hamas authorities were "exaggerated" and did not match information held by its military.
The latest strikes come just hours after reports of "heavy gunfire" from Israeli troops at the Indonesian Hospital in the city.
Rescue efforts in Beit Lahia are currently hampered due to communications and internet services being severed in the region, Gaza health officials said.
The Hamas-run government media office said the bombing hit "crowded" residential areas, and that 73 people had been killed - a number also reported by Gaza's civil defence agency. The BBC cannot independently verify the figures.
According to Palestinian news agency Wafa, an entire residential complex was destroyed in the strikes.
The Israel Defense Forces (IDF) told the BBC it had struck a "Hamas terror target" and was "doing everything possible to avoid causing harm to civilians".
It said the casualties given by the Hamas office were "exaggerated" and said such sources had "proven to be sorely unreliable in previous incidents."
Israel began a renewed military offensive in northern Gaza in early October, saying it is trying to prevent Hamas from regrouping in the area.
In particular, Israeli forces have surrounded and bombarded the densely-populated Jabalia area, which includes an urban refugee camp - with at least 33 people reported killed in a strike late on Friday.
Humanitarian groups have warned that virtually no aid has entered the area in the past few weeks. Israel's own statistics show that aid deliveries to Gaza as a whole have collapsed when compared with the same period in September.
The UN's top humanitarian official, Joyce Msuya, said on Saturday that Palestinians in northern Gaza are enduring "unspeakable horrors" and called for these "atrocities" to stop.
An Israeli minister, Amichai Chikli, told the BBC Israel had "blockaded" parts of northern Gaza.
"We allowed the civilian population to escape into the safe zone, and we prevented supplies to enter the blockade region," he told the Newshour programme - referring to the IDF having warned people in the north to evacuate.
He insisted this was "legal according to the international law".
Israel has repeatedly denied it is preventing humanitarian aid from entering Gaza but the US has told it to boost access or risk having some American military assistance cut off.
Meanwhile, Israel carried out at least a dozen air strikes on Beirut on Saturday, in the heaviest attack the Lebanese capital has witnessed in more than a week.
Damage and casualties were still being assessed, but at least one multi-storey building in the city's southern suburbs had been completely destroyed.
The Israeli military said it had targeted Hezbollah's weapons storage facilities and its intelligence headquarters command centre in Dahieh.
Smoke was seen rising from southern Beirut in the early hours of Sunday after Israel's military ordered the immediate evacuation of two neighbourhoods in the area.
The IDF told civilians they were "located near facilities and interests affiliated with Hezbollah, which the IDF will work against in the near future".