The World Health Organization said Sunday it was ready to pour much-needed aid into Gaza during the Israel-Hamas truce, but that it would need "systematic access" across the territory to do so.

Much of the Gaza Strip's health infrastructure has been destroyed by the more than year-long war between Israel and the Palestinian militant group Hamas before a cease-fire took hold on Sunday.

WHO chief Tedros Adhanom Ghebreyesus welcomed the cease-fire, posting on social media that it would "bring great hope for millions of people whose lives have been ravaged by the conflict."

But he added that "addressing the massive health needs and restoring the health system in Gaza will be a complex and challenging task, given the scale of destruction, operational complexity and constraints involved."

While the United Nations' health body was "ready to scale up the response" to address the territory's critical needs, it said in a statement that "it is critical that the security obstacles hindering operations are removed."

"WHO will need conditions on the ground that allow systematic access to the population across Gaza, enabling the influx of aid via all possible borders and routes, and lifting restrictions on the entry of essential items," the agency said in a statement.

Until the truce, Israel had complete control over the volume and nature of aid allowed into Gaza.

Warning that the "health challenges ahead are immense," the Geneva-based agency estimated the cost of rebuilding Gaza's battered health system in the years to come at "billions in investment".

Last week the WHO put the figure at more than $10 billion.

"Only half of Gaza's 36 hospitals remain partially operational, nearly all hospitals are damaged or partly destroyed, and just 38 percent of primary health care centers are functional," the WHO said.

WHO put the war's toll in Gaza at more than 46,600 people killed and over 110,000 wounded.

Gaza's health ministry placed the death toll at more than 46,913 Palestinians killed and 110,750 people wounded, figures that the U.N. considers reliable.

A quarter of those wounded "face life-changing injuries and will need ongoing rehabilitation," the U.N. body estimated.

Around 12,000 people need to be evacuated for urgent treatment elsewhere, it added, while warning the destruction of health infrastructure had had knock-on effects.

The WHO also expressed concern over the "breakdown of public order, exacerbated by armed gangs" interfering with aid deliveries to Gaza.