The World Health Organisation (WHO) found patients in agony as they received care on the floor on Saturday during a visit to a hospital in southern Gaza, the UN agency's chief said.
Tedros Adhanom Ghebreyesus called reports of ongoing hostilities and heavy bombardment in the territory "petrifying".
He said on social media platform X on Sunday that WHO's team had visited Nasser Hospital in Khan Younis the day before.
"It was packed with 1,000 patients – 3 times over its capacity. Countless people were seeking shelter, filling every corner of the facility," he added.
"Patients were receiving care on the floor, screaming in pain.
"These conditions are beyond inadequate – unimaginable for the provision of healthcare.
"I cannot find words strong enough to express our concern over what we’re witnessing," Adhanom Ghebreyesus said, calling for an immediate ceasefire.
A weeklong truce in the Gaza Strip broke down on Friday, and Israel is expected to expand its ground operations to the south.
Khan Younis, where the hospital is located, could be Israeli ground forces' next major target. Troops are currently only found in the north of the enclave, including Gaza City.
The Israeli military has called for areas in the east of Khan Younis including Bani Suhaila and Qarara to evacuate, calling the city a "dangerous combat zone" in a leaflet dropped to residents on Friday.
"The one who warns is excused," the flyer added.
Around two million people are now tightly packed into southern Gaza, following Israel's repeated orders for the evacuation of the north and ground offensive there.
Israel's indiscriminate military campaign in Gaza has so far killed over 15,200 people.
Hamas's surprise 7 October attack inside Israeli territory killed some 1,200 people.
Israel's air and ground assault has put many of the Palestinian territory's hospitals out of action.
An Israeli siege on Gaza, tightened after 7 October, has deprived hospitals of medical essentials including anaesthetics, as well as power need to run generators.