The World Health Organization said on Saturday that the Kamal Adwan hospital was "now empty" following a horrific Israeli raid which put north Gaza's last major health facility out of service.
The WHO said it was "appalled" by Friday's raid, saying "hospitals have once again become battlegrounds".
"The systematic dismantling of the health system and a siege for over 80 days on north Gaza puts the lives of the 75,000 Palestinians remaining in the area at risk," the UN health agency said in a statement.
Israel's military said on Saturday it had ended its raid on the facility, calling it "a Hamas command centre" though without evidence. The army also detained its director Hussam Abu Safiya, accusing him of being "an operative" for the group.
Israeli forces set fire to the hospital, burning down most of it. There were reports that some of the medical staff had been burnt alive.
Israeli forces began an intensified siege and ground operations in northern Gaza in October, under the so-called 'General's Plan', carried out with the aim of ridding the area of its Palestinian population and establishing a buffer zone.
"Kamal Adwan is now empty," the WHO said.
The remaining 15 critical patients, 50 caregivers and 20 health workers were transferred Friday to the Indonesian Hospital, which it described as "destroyed and non-functional".
"The movement and treatment of these critical patients under such conditions pose grave risks to their survival.
"WHO is deeply concerned for their wellbeing, as well as for the Kamal Adwan Hospital director."
The WHO said it had reports that the laboratory, surgical unit, engineering and maintenance department, operations theatre and medical store were all burned and severely damaged.
It said that earlier Friday, 12 patients had reportedly been forced to evacuate to the Indonesian Hospital.
"Additionally, some people were reportedly stripped and forced to walk toward southern Gaza.
"An urgent WHO mission to Indonesian Hospital is being planned for tomorrow to safely move patients to southern Gaza for continued care."
The WHO said the raid on Kamal Adwan had come following a period of escalating restrictions on its operations and repeated attacks.
It said only 10 out of 21 WHO missions to the hospital had been partially facilitated between early October and December, but the deployment of international emergency medical teams had been "repeatedly denied".
The UN health agency has verified at least 50 attacks on healthworkers at or near the hospital since early October this year. It never attributes blame for attacks.
"With Kamal Adwan and Indonesian hospitals entirely out of service, and Al-Awda Hospital barely able to function, and severely damaged due to recent airstrikes, the healthcare lifeline for those in north Gaza is reaching a breaking point," it said.
"WHO and partners' efforts to sustain the hospitals' operations have been undone.
"WHO calls for urgently ensuring that hospitals in north Gaza can be supported to become functional again."
The organisation said health facilities, workers and patients "must be actively protected and never be attacked, nor used for military purposes", but such calls "remain unheard".