The World Health Organization (WHO) has described Gaza City’s al-Shifa hospital as a “death zone” after visiting the site.
There has been widespread horror at reports of premature babies and intensive care patients left trapped inside al-Shifa after Israeli forces ordered an evacuation.
Hundreds of staff and patients reportedly fled, amid disputed claims soldiers gave them just an hour to leave.
Israeli forces have occupied al-Shifa hospital for more than 48 hours seeking to prove that Hamas has been using tunnels and command centres beneath the facility.
The Israel Defence Forces (IDF) had promised to "expose the terror activities within the hospital", as well as a "terror complex" below it.
A joint UN team led by the WHO assessed the hospital for one hour following its occupation and evacuation by the Israeli military.
The team said they saw evidence of shelling and gunfire and observed a mass grave at the hospital's entrance.
The UN said 291 patients were left at the hospital after Israeli troops told others to evacuate.
Those left included 32 babies in an extremely critical condition, trauma patients with severely infected wounds and others with spinal injuries who are unable to move.
The team was able to tour Shifa Hospital for an hour after about 2,500 displaced people, mobile patients and medical staff left the sprawling compound on Saturday morning, the WHO said.
It said 25 medical staff remained along with the patients.
"Patients and health staff with whom they spoke were terrified for their safety and health, and pleaded for evacuation," the agency said, describing Shifa as a death zone.
It said more teams will attempt to reach Shifa in the coming days to try to the evacuate patients to southern Gaza, where hospitals are also overwhelmed.
Hospital doctors deny militants are present and have called for an independent investigation.
Since occupying the hospital on Wednesday, Israeli soldiers claim to have discovered a small number of rifles and grenades at the complex.
On Thursday, the army published a photo of a hole in the ground it called a "Hamas terrorist tunnel".