Israel's military pounded Gaza on Saturday, a day after the International Committee of the Red Cross said 22 people were killed in shelling that damaged its office in the besieged Palestinian territory.
Exchanges of fire across the Lebanese border between Israel and the powerful Iran-backed militant group Hezbollah have also escalated, raising fears of an even wider war.
UN Secretary-General Antonio Guterres said the cross-border hostilities must not turn Lebanon into "another Gaza", warning of the risk of a catastrophe "beyond imagination".
His warning came as Israel stepped up its strikes in the Gaza Strip.
The fighting continued on Saturday, with witnesses reporting gun battles between militants and Israeli forces in Gaza City.
Witnesses also reported Israeli helicopters firing at militants in the city's Zeitun neighbourhood.
The Israeli military said Saturday its "fighter jets struck two Hamas military infrastructure sites in the area of Gaza City".
Witnesses said the strikes hit the Al-Shati camp and Draj Tufah neighbourhood. They said at least four residential buildings and nearby houses were destroyed.
Earlier, the military said troops had continued to carry out operations in central Gaza and that its jets had struck targets across the Palestinian territory.
- 'A difficult and brutal day' -
The ICRC said late Friday 22 dead and 45 wounded people were taken to a Red Cross field hospital after shelling with "heavy calibre projectiles" near its office in southern Gaza.
"Firing so dangerously close to humanitarian structures puts the lives of civilians and humanitarians at risk," the ICRC said on X.
The health ministry in the Hamas-run territory blamed the shelling on Israel, saying 25 were killed and 50 wounded in the southern coastal Al-Mawasi area, where thousands of displaced have been sheltering in tents.
An Israeli military spokesman did not elaborate on the incident but said it was "under review".
In the territory's north, Gaza City's Al-Ahli hospital director Dr Fadel Naeem was quoted by the health ministry as reporting 30 dead in strikes on "a difficult and brutal day" Friday.
- 'Catastrophe beyond imagination' -
Lebanon-based Hamas ally Hezbollah meanwhile claimed a number of attacks on Israeli troops and positions near the border on Friday, including two using drones.
Israel said it had carried out multiple retaliatory strikes.
Jets on Friday hit a "Hezbollah military structure in the area of Khiam, a Hezbollah military post in the area of Mais al-Jabal, and Hezbollah terrorist infrastructure in the areas of Taybeh and Tallouseh in southern Lebanon", an army statement said.
Experts are divided on the prospect of a wider war, almost nine months into Israel's campaign to eradicate Hamas in the Gaza Strip.
Amid the escalating exchanges with Hezbollah, Israel's military said Tuesday that plans for an offensive in Lebanon had been "approved and validated".
Hezbollah chief Hassan Nasrallah said "no place" in Israel would "be spared our rockets" in a wider war, and also threatened nearby European Union member Cyprus.
Citing "bellicose rhetoric" on both sides, Guterres warned Friday that the risk of all-out war was real.
"One rash move –- one miscalculation -- could trigger a catastrophe that goes far beyond the border, and frankly, beyond imagination," he said.
- 'Nothing left' -
The violence on the Lebanon border began after the October 7 attack on southern Israel by Hamas militants from Gaza. That attack resulted in the deaths of 1,194 people, mostly civilians, according to an AFP tally based on Israeli official figures.
The militants also seized hostages, 116 of whom remain in Gaza although the army says 41 are dead.
Israel's retaliatory offensive had killed at least 37,551 people, also mostly civilians, Gaza's health ministry said on Saturday,reporting at least 120 deaths over the past 48 hours.
Months of negotiations towards a truce and a hostage release have failed to make headway, but mediator Qatar insisted Friday it was still working to "bridge the gap" between Israel and Hamas.
The war has destroyed much of Gaza's infrastructure and left residents short of food, fuel and other essentials.
On June 16 the army said it would implement a daily "tactical pause of military activity" in a southern Gaza corridor to facilitate aid delivery.
But on Friday Richard Peeperkorn of the World Health Organization said "we did not see an impact on the humanitarian supplies coming in".
"We don't see any aid," said Umm Mohammad Zamlat, 66, from northern Gaza but now living in Khan Yunis in the south.
"Even agencies specialised in aid deliveries are not able to provide anything to us," she added.
Doctor Thanos Gargavanis, a WHO trauma surgeon and emergency officer, said Friday the UN in Gaza was trying to "operate in an unworkable environment".
According to the WHO, 17 of the 36 hospitals in Gaza are operational, but only partially.