The World Health Organization said it was forced to postpone the last phase of the polio vaccination drive in Gaza on Wednesday due to "intense bombardment" and "escalating violence" in the north.
The second round of vaccinations was due to begin in the north of the Gaza Strip on Wednesday, after having already been completed in the central and southern sections of the besieged Palestinian territory.
But the WHO said it had been "compelled to postpone" the campaign to vaccinate 119,279 children in northern Gaza.
Israel launched a major air and ground assault in northern Gaza this month, vowing to stop Hamas militants from regrouping in the area.
The 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 WHO said.
"The current conditions, including ongoing attacks on civilian infrastructure, continue to jeopardise 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.
Most often 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.
As during the initial round of vaccination last month, the second round was divided into three phases, helped by localised "humanitarian pauses" in the fighting: first in central Gaza, then in the south and finally in the hardest-to-reach north.
Each phase was scheduled to take three campaign days, along with one catch-up day for monitoring and for vaccinating any children who were missed.
The WHO says a minimum of two doses of oral vaccine are needed to interrupt poliovirus transmission, requiring 90 percent of all children aged under 10 to be vaccinated in a given community.
"It is imperative to stop the polio outbreak as soon as possible, before more children are paralysed 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, ensuring access for wherever eligible children are located."
