The assessment came after a visit by WHO and other UN officials to the hospital, which Israeli troops raided earlier this week.
Elsewhere, a Palestinian health official said more than 80 people were killed Saturday in twin strikes on a northern Gaza refugee camp, including on a UN school sheltering displaced people.
Videos verified by AFP showed bodies covered in blood and dust on the floor of a building where mattresses had been wedged under school tables, in Jabalia, the Palestinian territory's biggest refugee camp.
Philippe Lazzarini, head of the UN Agency for Palestinian Refugees UNRWA, described "horrifying images" from the incident, while Egypt called the bombing a "war crime" and "a deliberate insult to the United Nations".
A separate strike Saturday on another building in Jabalia camp killed 32 people from the same family, 19 of them children, Palestinian health authorities said.
Without mentioning the strikes, the Israeli army said "an incident in the Jabalia region" was under review.
The UN says some 1.6 million people have been displaced inside the Gaza Strip by six weeks of fighting, and Israel said Saturday its military was now "expanding its operational activities in additional neighbourhoods in the area of the Gaza Strip".
Gaza's largest hospital, Al-Shifa, has been a key focus in recent days, and on Sunday, the WHO described the hospital as a "death zone", with a mass grave at the entrance and nearly 300 patients left inside with 25 health workers.
It said it was planning "the immediate evacuation of the remaining patients, staff and their families", warning, however, that nearby facilities were already overstretched and urging an immediate ceasefire given the "extreme suffering of the people of Gaza".
On Saturday, hundreds of people fled the hospital on foot on orders from the Israeli army, according to the facility's director.
Columns of sick and injured -- some of them amputees -- were seen leaving with displaced people, doctors, and nurses, as loud explosions were heard around the complex.
At least 15 bodies, some in advanced stages of decomposition, were strewn along the route, lined with heavily damaged shops and overturned vehicles, an AFP journalist there said.
Doctors without Borders said a convoy carrying its staff and family members came under attack Saturday while evacuating from near Al-Shifa, despite coordinating with both sides. One person was killed.
The WHO said 29 patients at the hospital with serious spinal injuries cannot move without medical assistance, and others have infected wounds due to lack of antibiotics.
There are also 32 babies in "extremely critical condition," WHO said.
Israel's siege on Gaza has left food, water, medicine and fuel in short supply, with just a trickle of aid allowed in from Egypt.
Under US pressure, Israel permitted a first consignment of fuel to enter late Friday, allowing telecommunications to resume after a two-day blackout.
The UN said Israel had agreed to allow in 60,000 litres (16,000 gallons) of fuel a day from Saturday, but warned it only around a third of what is needed.
Israel has told Palestinians to move south for their safety, but deadly strikes continued there too. At least 26 people were killed in a residential building on Saturday, according to the director of the Nasser Hospital in Khan Yunis.
Diplomacy to secure the release of hostages is continuing, with a US official saying more fuel deliveries and a "significant pause" in fighting would come "when hostages are released".
The White House denied, however, a Washington Post report of a tentative agreement, with National Security Council spokeswoman Adrienne Watson saying "We continue to work hard to get to a deal".
Gaza's fate after the conflict remains unclear, and Biden argued in an opinion piece published Saturday that the coastal territory and the Israeli-occupied West Bank should come under a single "revitalised" administration.
"As we strive for peace, Gaza and the West Bank should be reunited under a single governance structure, ultimately under a revitalised Palestinian Authority," he wrote in the Washington Post.
However, Netanyahu has insisted the Palestinian Authority "in its current form is not capable of receiving responsibility for Gaza".
Biden also threatened sanctions, including visa bans, against settlers who have ramped up attacks on Palestinians in the West Bank in recent weeks.