The WHO chief said its team had seen "at least five dead bodies during the mission".
The World Health Organization said Saturday that Gaza's largest hospital had been reduced to ashes by Israel's latest siege, leaving an "empty shell" with many bodies.
Israeli forces pulled out of Al-Shifa hospital in Gaza City on Monday after a two-week military operation, during which it said it had battled Palestinian operatives inside what was once the Palestinian territory's most important medical complex.
A WHO-led mission finally accessed the hospital on Friday, after multiple failed attempts since March 25, the UN health agency said, describing the massive destruction.
"WHO and partners managed to reach Al-Shifa -- once the backbone of the health system in Gaza, which is now an empty shell with human graves after the latest siege," agency chief Tedros Adhanom Ghebreyesus wrote on X, formerly Twitter.
He said the team had seen "at least five dead bodies during the mission".
They had also found that "most of the buildings in the hospital complex are extensively destroyed and the majority of assets damaged or reduced to ashes", the WHO chief said.
"Even restoring minimal functionality in the short term seems implausible," he said, adding that "an in-depth assessment by a team of engineers is needed to determine if the remaining buildings are safe for future use".
Tedros lamented that efforts by WHO and other aid groups to revive basic services at Al-Shifa after Israel's first devastating raid on the hospital last year "are now lost, and people are once again deprived of access to lifesaving health care services".
Of Gaza's 36 main hospitals, only 10 remain partially functional, according to WHO.
The Gaza war began on October 7 with an unprecedented attack cross-border attack by Hamas militants resulting in the death of 1,170 people in southern Israel, mostly civilians, according to an AFP toll from official Israeli figures.
Palestinian militants also took around 250 hostages, about 130 of whom remain in Gaza. The army says that more than 30 are dead.
Vowing to destroy Hamas, Israel has relentlessly bombarded the territory, killing at least 33,137 people, mostly women and children, according to the health ministry in the Hamas-run territory.
Tedros said urgent action was needed in Gaza as "famine looms, disease outbreaks spread and trauma injuries increase".
He called for the "protection of remaining health facilities in Gaza (and) protection of health and humanitarian workers".
The WHO chief demanded "unimpeded access of humanitarian aid into and across the Gaza Strip" and a "ceasefire".
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