The World Health Organization has confirmed the first case of polio in war torn Gaza.
It said that a ten-month baby has been left paralysed due to polio, in the first confirmed case in more than 25 years.
Last month, the WHO said polio had been detected in Gaza and warned that children there would soon be infected by the disease if preventative measures were not quickly taken.
Poliomyelitis, which is spread mainly through the faecal-oral route, is a highly infectious virus that can invade the nervous system and cause paralysis and death in young children.
Polio cases have declined by 99% worldwide since 1988 thanks to mass vaccination campaigns and efforts to eradicate it.
In a post on X, UNRWA chief Philippe Lazzarini said polio "will not make the distinction between Palestinian & Israeli children. Delaying a humanitarian pause will increase the risk of spread among children".
UNRWA said it teams will vaccinate children in "both our primary healthcare centres and mobile clinics starting at the end of the month".
UN Director General Tedros Adhanom Ghebreyesus said he was "gravely concerned" at news of the first polio case in Gaza.
In a post on X, Dr Tedros said: "Genomic sequencing confirmed the virus is linked to the variant poliovirus type 2 detected in environmental samples collected in June from Gaza's wastewater. The child, who developed paralysis in the lower left leg, is currently in a stable condition".
He added: "Given the high risk of poliovirus spread in Gaza and the region, the Palestinian Ministry of Health,@WHO and @UNICEF are working to implement two rounds of polio vaccination in the coming weeks to halt transmission."
Gaza's first recorded polio case has health workers and aid agencies grappling with the steep obstacles to conducting mass vaccination in the territory.
Unrelenting air strikes by Israel, restrictions of aid entering the besieged territory and hot summer temperatures all threaten the viability of a life-saving inoculation drive.
Still, equipment to support the extensive campaign - which UN agencies say could start on 31 August - has already arrived in the region.
The WHO and children's agency UNICEF say the have detailed plans to vaccinate 640,000 children across Gaza.
But a major challenge remains Israel's devastating military campaign.
"It's extremely difficult to undertake a vaccination campaign of this scale and volume under a sky full of air strikes," said Juliette Touma, director of communications for the UN agency for Palestinian refugees, UNRWA.
Under the UN plan, 2,700 health workers in 708 teams would take part, with the WHO overseeing the effort, said Richard Peeperkorn, the agency's representative in the Palestinian territories.
UNICEF would ensure the cold supply chain as vaccines are brought into and distributed across Gaza, spokesman Jonathan Crickx said.
Cold chain components including refrigerators arrived on Wednesday at Israel's main international airport.
Some 1.6 million doses of the oral vaccine would follow, and are expected to enter Gaza on Sunday via the Kerem Shalom crossing, Mr Crickx said.
The UN agencies plan to administer two doses each for about 95% of children under 10 in Gaza, according to Mr Crickx. Surplus doses would cover expected losses to heat or other causes.
Ms Touma, who worked on polio response during wars in Iraq and Syria, said that "the return of polio to a place where it's been eradicated says quite a lot."
Gaza's health care system has been decimated, with "only 16 out of 36 hospitals... still functioning, and only partially," Mr Crickx said.
Out of those, only 11 facilities are capable of maintaining the cold chain, he added.
The vaccines would first be kept at a UN storage space in central Gaza, and then distributed to public and private health facilities as well as UNRWA shelters "hopefully by refrigerated trucks if we can find some, otherwise by cold boxes" filled with ice packs, he said.
Many Gazans now live in makeshift camps or UNRWA schools, making them hard to reach, said Moussa Abed, director of primary health care at the Gaza health ministry.
Nearly all of the territory's 2.4 million people have been displaced at least once during the war.
UN Secretary-General Antonio Guterres has called for two seven-day breaks in the war to administer doses.
