(AFP) - The WHO on Wednesday said that a ceasefire between Israel and Hamas was possible but those with influence were not doing enough to secure it, expressing concern at Gaza's "crippled" health system.

World Health Organization chief Tedros Adhanom Ghebreyesus said there was a high chance of fighting resuming, with the current truce scheduled to expire early Thursday after a six-day pause in the conflict.

The UN health agency is calling for the release of hostages held in the Gaza Strip, and for a sustained ceasefire so that aid can continue to be delivered into the Palestinian enclave to relieve further civilian suffering.

"I really believe that the humanitarian pause, or even the ceasefire, is possible if those with influence can take it seriously," Tedros told a press conference.

"It's possible except those with influence are not doing it. That's the situation. It can happen; it's a matter of will."

Tedros said the pause in hostilities had allowed the WHO to increase deliveries of medical supplies into Gaza and transfer patients from Al-Shifa hospital, the largest in the territory, to other hospitals in the south of the strip.

During the first three days of the truce, the WHO received 121 pallets of supplies into its warehouse in Gaza, including intravenous fluids, medicines, laboratory supplies and trauma and surgical supplies.

"WHO's greatest concern remains supporting Gaza's health system and health workers to function," Tedros said, describing the health system as "already broken".

Only 15 of Gaza's 36 hospitals are still functioning with any capacity, but are "completely overwhelmed".

And of the 25 hospitals in the north, only three are still operating at the most basic level, he added, and lacking water, fuel and food.

- 'Gargantuan' challenge' -

Richard Peeperkorn, the WHO representative in the occupied Palestinian territories, added: "We are extremely concerned about the vulnerability of what I call a crippled health system."

He said Gaza had 3,500 hospital beds before the war but only 1,500 were still available and the bed capacity needed to be expanded immediately, within those hospitals still functioning.

"We estimate there's a need of 5,000 beds so we really have a long way to go," he said.

"Any resumption of violence could damage the health facilities and make more health facilities dysfunctional. Gaza can absolutely not afford to lose more hospital beds."

WHO emergencies director Michael Ryan said there was "no way" that incoming emergency medical teams could replace the lost capacity in the Gaza health system.

"Tomorrow, even if peace was declared, we have a massive challenge ahead of us: an absolutely gargantuan public health and health delivery challenge," he said.

The truce agreement has brought a temporary halt to fighting that began on October 7 when Hamas militants poured over the border into Israel, killing 1,200 people, mostly civilians, and kidnapping about 240.

Israel's subsequent air and ground campaign in Gaza has killed nearly 15,000 people, also mostly civilians, according to Hamas officials, and reduced large parts of the north of the territory to rubble.

© Agence France-Presse