Gaza's main Al Shifa hospital, where thousands of people are sheltering inside, is no longer functioning amid heavy airstrikes and shelling, the World Health Organisation (WHO) has warned.
"The situation is dire and perilous," WHO director-general Tedros Adhanom Ghebreyesus said on Sunday evening.
The enclave's main hospital has suffered three days without electricity, water and with "very poor internet", he wrote on X.
"The constant gunfire and bombings in the area have exacerbated the already critical circumstances. Tragically, the number of patient fatalities has increased significantly.
"The hospital is not functioning as a hospital anymore."
Mr Ghebreyesus added: "The world cannot stand silent while hospitals, which should be safe havens, are transformed into scenes of death, devastation, and despair. Ceasefire. NOW."
His plea comes after Israeli Prime Minister Benjamin Netanyahu claimed his country had offered fuel to Gaza's Al Shifa hospital but Hamas militants refused to receive it.
The hospital's last generator reportedly ran out of fuel on Saturday, leading to the deaths of three premature babies and several other patients, according to the Hamas-run Health Ministry. It said another 36 babies are at risk of dying without electricity.
On Sunday, Mr Netanyahu told NBC News: "We offered... to give them enough fuel to operate the hospital, operate the incubators and so on, because we (have) no battle with patients or civilians at all.
"We just offered Shifa hospital the fuel, they refused it.
"Hamas, which is hiding in the hospitals and placing itself there, doesn't want the fuel for the hospital ... they want to get fuel that they'll take from the hospitals to their tunnels, to their war machine."
Newborns reportedly taken off incubators in Gaza's Al Shifa hospital after power outage
Health officials and people trapped inside Gaza's largest hospital rejected Israel's claims that it was helping babies and others evacuate Sunday, saying fighting continued just outside the facility.
Mr Netanyahu has dismissed urgent calls for a ceasefire unless it includes the release of all the nearly 240 hostages captured by Hamas in the October 7 rampage.
A day after he said Israel was bringing its "full force" to Gaza, residents reported heavy airstrikes and shelling, including around Shifa Hospital.
Israel, without providing evidence, has accused Hamas of concealing a command post inside and under the compound, allegations denied by Hamas and hospital staff.
Israel's military asserted it placed 300 liters (79 gallons) of fuel near Shifa overnight for an emergency generator powering incubators for premature babies and coordinated the delivery with hospital officials. But the military said Hamas prevented the hospital from receiving the fuel.
Smoke rises as displaced Palestinians take shelter at Al Shifa hospital
A Health Ministry spokesperson, Ashraf al-Qidra, told Al Jazeera: "This is a mockery towards the patients and children."
Speaking to CNN, Mr Netanyahu asserted that "100 or so" people had been evacuated from Shifa and that Israel had created safe corridors.
But Health Ministry Undersecretary Munir al-Boursh said Israeli snipers have deployed around Shifa, firing at any movement.
"There are wounded in the house, and we can't reach them," he told Al Jazeera. "We can't stick our heads out of the window."
Medical Aid for Palestinians, a UK-based charity that has supported Shifa's neonatal intensive care unit, said transferring critically ill infants is complex.
"With ambulances unable to reach the hospital ... and no hospital with capacity to receive them, there is no indication of how this can be done safely," CEO Melanie Ward said.
The only option is for Israel to stop its assault and allow fuel into the hospital, Ms Ward said.
The Health Ministry said there are 1,500 patients at Shifa, along with 1,500 medical personnel and between 15,000 and 20,000 people seeking shelter.
Patients rest at Al-Shifa hospital in Gaza City
The Palestinian Red Crescent rescue service said another Gaza City hospital, Al-Quds, is "no longer operational" because it was out of fuel with 6,000 people trapped there. Gaza's sole power plant shut down a month ago, and Israel has barred fuel imports to prevent Hamas from using them.
About 2.3 million Palestinians remain in the besieged territory. The war has displaced over two-thirds of Gaza's population.
Mr Netanyahu has said the responsibility for any harm to civilians lies with Hamas.
The US has pushed for temporary pauses that would allow for wider distribution of desperately needed aid to civilians in the territory.
But Israel has only agreed to brief daily periods during which civilians can flee ground combat in northern Gaza and head south on foot along two main roads. Israel continues to strike what it says are militant targets across southern Gaza, often killing women and children.
Wael Abu Omar, spokesperson for Gaza's border crossings, said 846 people left Gaza to Egypt through the Rafah crossing Sunday. He said 76 aid trucks entered Gaza.
More than 11,000 Palestinians, two-thirds of them women and minors, have been killed since the war began, according to the Health Ministry in Gaza, which does not differentiate between civilian and militant deaths. About 2,700 people have been reported missing.
At least 1,200 people have been killed in Israel, mostly civilians killed in the initial Hamas attack. The Israeli military said two more of its soldiers were killed in Gaza, bringing the total to 48.
About 250,000 Israelis have evacuated from communities near Gaza, where Palestinian militants are still firing barrages of rockets, and along the northern border with Lebanon.