Gaza's health system is on its knees and cannot afford to lose another ambulance or a single hospital bed more, the World Health Organization warned on Friday.

"The situation is getting more and more horrible by the day... beyond belief, literally," WHO spokesman Christian Lindmeier told a press briefing in Geneva.

"The health system is on its knees. Gaza cannot afford to lose any more health facilities, another single ambulance, any more hospitals... or even a single hospital bed more."

The United Nations' humanitarian agency OCHA said late on Thursday that only 14 of the 36 hospitals in the Gaza Strip were functioning in any capacity.

War erupted in Gaza after Hamas militants attacked southern Israel on October 7, killing about 1,200 people and taking hostages, 138 of whom remain captive, according to Israeli figures.

Israel's retaliatory air and ground assault on Gaza has killed 17,177 people, mostly women and children, according to the Hamas-run health ministry in the besieged Palestinian territory.

Weeks of fighting have reduced vast areas of Gaza to wasteland.

The UN says about 80 percent of the population has been displaced, facing shortages of food, fuel, water and medicine, along with the threat of disease.

"Children and people begging and crying for water -- we're at that level, where the most normal and basic supplies are not available any more," Lindmeier said.

"Right now the calculation for Gaza is one to two litres of fresh water a day -- that's water for everything, not only for drinking.

"People are starting to cut down telephone poles to have a little bit of firewood to keep warm or maybe cook, if they have anything available.

"Civilisation is about to break down."

Lindmeier said a convoy was supposed to take medical supplies to Al-Ahli Arab hospital in Gaza City on Friday and evacuate 12 patients to the south.

"That mission, we were told this morning, had to be suspended because of the security situation," he said.

Health workers in the Gaza Strip do not have enough food and water to continue working, he said.

"Patients are bleeding on the floor, trauma wards resemble battlefields," said Lindmeier.

"This callousness must end. We need a ceasefire and we need it now."