Sean Casey, a WHO emergency medical teams coordinator
WHO staff described desperate scenes of seriously injured patients, including young children, begging for food in hospitals, which have seen most of their health workers flee for their safety.
"We're seeing this humanitarian catastrophe unfold before our eyes," Sean Casey, a WHO emergency medical teams coordinator, told reporters in Geneva via video link from the Gaza Strip.
"We're seeing the health system collapse at a very rapid pace," he warned.
The Israeli army has claimed the war is entering a new phase, involving troop reductions and more targeted operations in the territory's centre and south.
But Casey said that on the ground, he had "not seen the lowering of the intensification".
"What we are still seeing... is a huge number of casualties related to hostilities, so shrapnel injuries, gunshot wounds, crush injuries from buildings that collapse. That's still happening every single day."
Israel's relentless bombardment and ground invasion of the Gaza Strip in three months have killed more than 23,200 people, 70 percent of them women and children, with another 59,167 injured according to the Palestinian health ministry's latest figures.
'Recipe for disaster'
The United Nations says the war has displaced around 85 percent of Gaza's population of 2.4 million, and left civilians in the besieged Palestinian territory at risk of famine and disease.
A UN Security Council resolution last month demanded that more aid be let in but the WHO said its access had only got worse.
"We've seen the shrinking of humanitarian space," Casey said.
Israel has claimed the United Nations is largely to blame for the lack of aid reaching those in need in Gaza.
But Casey insisted the WHO and other UN organisations were "constantly trying to reach the areas in greatest need".
"Every day we line up our convoys, we wait for clearance and we don't get it," he said.
"And then we come back and we do it again the next day."
The WHO has been unable to reach northern Gaza for the past two weeks and has been forced to cancel six planned missions there.
The organisation said that only 15 of Gaza's 36 hospitals are even partially functioning, most of them in the south.
The agency has long described desperate scenes in the few barely functioning hospitals remaining in the north, facing severe shortages of food, clean water, medicines and fuel.
And it warned that the situation was increasingly dire in the middle and south of the densely populated territory.
"Hostilities and evacuation orders in neighbourhoods of the middle area and Khan Younis... are affecting access to hospitals for patients and ambulances, and making it incredibly complex for WHO to reach those hospitals to provide supplies and fuel," said Richard Peeperkorn, WHO's representative for the Palestinian territories.
Speaking to journalists from Jerusalem, he warned that this "was a recipe for disaster and will make more hospitals non-functional".
'Must be protected'
The European Gaza Hospital, Nasser Medical Complex and Al-Aqsa Hospital in the middle area, long among the best functioning facilities, are now near evacuation zones, Casey pointed out.
"We cannot lose these health facilities," he said.
"They absolutely must be protected."
He visited the Al-Aqsa hospital on Sunday, finding that hundreds of patients and around 70 percent of health workers had fled for safety amid increasing hostilities around the facility.
The few remaining staff were struggling to care for patients lying on blood-streaked floors.
"It was mostly children with gunshot wounds, with shrapnel injuries. Children who were playing in the streets when the building next to them exploded," Casey said.
* This story was edited by Ahram Online.
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