Gaza - The World Health Organization said Wednesday that "intense bombardment" and "escalating violence" in northern Gaza had forced it to postpone the final phase of a child polio vaccination drive.

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, sparking fears for the hundreds of thousands of Palestinians trapped under increasingly desperate conditions.

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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 UN 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 Israeli siege 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.

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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 assault: 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 UN 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 October 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 UN health agency said.

"It is crucial therefore that the vaccination campaign in northern Gaza is facilitated through the implementation of the humanitarian pauses."

Israel has killed more than 42,700 people in Gaza, according to figures from the territory's health ministry. The British medical journal Lancet and other experts believe the true number to be far greater, potentially upwards of 186,000 as of July.