The World Health Organization announced Wednesday that escalating Israeli violence and heavy bombardment in northern Gaza had delayed the final phase of a child polio vaccination campaign.
The necessary second round of vaccinations has been completed in central and southern Gaza and was to begin on Wednesday in the north.
But the WHO said it had been "compelled to postpone" the bid to give 119,279 children in northern Gaza a second vaccine dose.
Israel launched a major air and ground assault in northern Gaza this month, alleging Hamas was regrouping in the area.
The vaccination campaign was called off "due to the escalating violence, intense bombardment, mass displacement orders and lack of assured humanitarian pauses across most of northern Gaza," the U.N. health agency said.
"The current conditions, including ongoing attacks on civilian infrastructure, continue to jeopardize people's safety and movement in northern Gaza, making it impossible for families to safely bring their children for vaccination," and for health workers to operate, it added.
The vaccination drive began after the Gaza Strip confirmed its first case of polio in 25 years.
The war has left most medical facilities and Gaza's sewage system in ruins.
Typically spread through sewage and contaminated water, poliovirus is highly infectious.
It can cause deformities and paralysis, and is potentially fatal, mainly affecting children under the age of five.
The WHO says a minimum of two separate doses of oral vaccine are needed to interrupt poliovirus transmission, requiring 90% of all children aged under 10 to be vaccinated in a given community.
As in the initial round of vaccinations last month, the second round was divided into three phases, helped by localized "humanitarian pauses" in the fighting: first in central Gaza, then in the south and finally the north.
Each phase was to take up to four days.
The WHO warns that immunity levels are lower if the second dose is given more than six weeks after the first.
The U.N. agency said the approved area for humanitarian pauses in the north had been cut down to Gaza City alone, meaning many children would have missed the second dose.
This would "seriously jeopardize efforts to stop the transmission of poliovirus in Gaza," it said.
Since the second round of the campaign began on Oct. 14, some 442,855 children under 10 have been vaccinated in central and southern Gaza, with coverage at 94%.
Meanwhile, 357,802 children aged two to 10 were given vitamin A supplements.
"It is imperative to stop the polio outbreak as soon as possible before more children are paralyzed and poliovirus spreads further," the U.N. health agency said.
"It is crucial therefore that the vaccination campaign in northern Gaza is facilitated through the implementation of the humanitarian pauses."
The war in Gaza began with Hamas' Oct. 7, 2023 attack on Israel, which resulted in the deaths of 1,206 people, mostly civilians, according to an AFP tally of Israeli official figures.
Israel's retaliatory offensive has killed more than 42,700 people in Gaza, also mostly civilians, according to figures from the Hamas-run territory's health ministry which the U.N. considers reliable.
