Even before fighting resumed in Gaza on Friday after a week-long truce, its health system was on its knees, with hospitals resembling a "horror movie", the World Health Organization said.

As Israeli bombs began raining down on the besieged Palestinian territory again, WHO officials inside Gaza warned that the healthcare situation there was already "catastrophic".

"We are extremely concerned about the resumption of violence," Richard Peeperkorn, WHO's representative in the Palestinian territories, told reporters in Geneva via video-link from Gaza.

Already, he said, "the health system in Gaza has been crippled by the ongoing hostilities".

"It cannot afford to lose more hospitals."

Intense fighting erupted again early on Friday, immediately after the expiry of a week-long truce between Israel and Hamas.

The truce had paused fighting that began on October 7 when Hamas militants broke through Gaza's militarised border into Israel, killing around 1,200 people, mostly civilians, and kidnapping about 240 others, according to Israeli authorities.

Israel responded by vowing to eliminate Hamas and unleashing a relentless air and ground offensive in Gaza that has killed more than 15,000 people in Gaza, including more than 6,000 children, according to the Hamas-run government there.

- 'Awash with blood' -

Currently, only 18 of Gaza's 36 hospitals are even minimally to partially functional, with the three main hospitals in the north barely operative, Peeperkorn said.

In the south of Gaza, 12 hospitals are partially functional but are "extremely overwhelmed", he said, pointing to several that currently have twice or three times as many patients as they have beds.

All the hospitals were suffering shortages of supplies, fuel, food and even water, he said, and there was an "urgent need" for a sustained flow of aid "to keep them functional".

Hospital bed capacity across Gaza has fallen from 3,500 to just 1,562 even as needs have skyrocketed, he said, adding that at least 5,000 beds were needed.

"The healthcare service is on its knees," said Rob Holden, a WHO senior emergency officer, also speaking from Gaza.

He said he and other WHO staff had in recent days visited the Al Ahli Hospital in Gaza City -- a relatively small hospital which suddenly finds itself the only receiving centre for trauma patients in the north.

"It is like a horror movie," he said via video-link, with the sound of explosions in the background.

He described patients covering the floor "with the most traumatic injuries that you can imagine".

"There is no standing room. The floor is just awash with blood and patients lying waiting to receive life-saving care."

Patients were being given the best possible care, he said, but with many staff having been killed or having fled with their families, there were not enough people to care for them and not enough supplies.

At the same time, staff were struggling to deal with the growing number of dead.

"It is done with the best possible dignity but still the bodies are lined up in the car park outside," Holden said.

He said he woke up on Friday morning as the bombs began falling again, and thought of the doctors he had spent the last couple of days with, "knowing full well what they will face".

Those medics, he said, would be grappling with "the most horrific scene, as casualties probably in their tens if not hundreds turn up at that hospital today, knowing that they will do everything they can, but knowing that it will not be enough".

"The bombs have got to stop."