UN agencies say they have postponed an emergency polio vaccination campaign in northern Gaza due to intense Israeli bombardments, mass displacement and lack of access.

The final phase of the two-stage rollout – prompted by Gaza’s first case of polio in 25 years, which left a baby boy paralyzed – was due to begin on Wednesday.

Almost 120,000 children in northern Gaza had been expected to receive a second dose of the oral polio vaccine.

But Israel’s military has been carrying out a new offensive in the Jabalia area, saying that it is acting against regrouping Hamas fighters.

More than 400 people are reported to have been killed and 60,000 others have been displaced since the offensive started two weeks ago.

Residents unwilling or unable to comply with Israeli evacuation orders are said to be living in increasingly desperate conditions, with food and other essential supplies running out.

From the start of the polio vaccination campaign in Gaza, medical experts stressed that delays in administering the second dose could jeopardise overall efforts to halt transmission of the contagious, potentially deadly disease.

For herd immunity to work, at least 90% of all children in every community and neighbourhood need to be given a minimum of two doses.

The first round of the vaccine campaign successfully reached 559,000 children under 10 years of age over three phases in southern, central and northern Gaza between 1 and 12 September, during which there were local “humanitarian pauses” agreed by Israel and Palestinian groups.

Following the start of the second round on 14 October, more than 442,000 children in central and southern areas received the second dose – 94% of the target in those areas.

However, UN agencies said in a joint statement on Wednesday that they had been compelled to postpone the third phase to vaccinate 119,000 children across the north.

“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 health workers to operate,” it said.

“All logistics, supplies and trained human resources were prepared,” they added. “However, given that the area currently approved for temporary humanitarian pauses was substantially reduced – now limited only to Gaza City, a significant decrease from the first round – many children in northern Gaza would have missed out on the polio vaccine dose.”

The UN agencies said it was crucial for the vaccine campaign in the north to be facilitated through the implementation of humanitarian pauses that would ensure access for wherever children are located.

The Israeli military body in charge of co-ordinating aid shipments to Gaza, Cogat, said the vaccination campaign would begin in the coming days. “We will continue to facilitate an effective vaccination campaign against polio across Gaza,” it added.

The World Health Organization’s representative in Gaza also told the BBC that he was hopeful that the campaign would start within the next week.

“We cannot stop this now. We are almost there,” Dr Richard Peeperkorn said.

He also described how he had witnessed “enormous destruction” and “active bombardment all over the place” earlier this week during a WHO mission to evacuate 14 patients from Kamal Adwan hospital in Beit Lahia, one of the two minimally functional hospitals near Jabalia.

“We had to stay overnight at Kamal Adwan. There was intense bombardment in the vicinity of the hospital. What these patients go through every night,” he said.

“I could already see that we could not do a polio vaccine campaign in those conditions, in that environment. Parents would not be able to bring their children, teams would not be able to move around.”

Dr Peeperkorn said the hospital had received hundreds of injured people and many dead bodies over the last couple of days.

On Wednesday morning, the Israeli military said its troops had killed “numerous terrorists” and located weapons during continuing operations in the Jabalia area.

It also said the troops had “apprehended dozens of terror operatives while facilitating the safe evacuation of thousands of civilians”.

Gaza’s Hamas-run Civil Defence agency meanwhile said its rescue workers had recovered the bodies of two people from a building destroyed in an Israeli strike in the Beit Lahia Project area overnight. Two children and their mother were also killed at a home in al-Zarqa area, north of Gaza City, it added.

Later on Wednesday, the Civil Defence reported that a number of people were killed in a strike on school sheltering displaced people in Gaza City. Graphic video posted online appeared to show the bodies of several men lying in the courtyard of al-Zahraa school.

The Israeli military said it had conducted “a precise strike on Hamas terrorists who were operating inside a command-and-control centre”.

Meanwhile, a driver for the UN agency for Palestinian refugees (Unrwa) was killed in a strike on a UN-marked lorry in the central town of Deir al-Balah.

There was no immediate comment from either Unrwa or the Israeli military.