The United Nations, in collaboration with Palestinian health authorities, has begun vaccinating 640,000 children in Gaza, with Israel and Hamas agreeing to brief pauses in their 11-month war to allow the campaign to go ahead.
The World Health Organization (WHO) confirmed last month that a baby was partially paralysed by the type 2 polio virus, the first such case in the territory in 25 years.
The campaign began today in areas of central Gaza, and will move to other areas in coming days.
Fighting will pause for at least eight hours on three consecutive days.
The WHO said the pauses will likely need to extend to a fourth day and the first round of vaccinations will take just under two weeks.
"This is the first few hours of the first phase of a massive campaign, one of the most complex in the world," said Juliette Touma, communications director of UNRWA, the UN Palestinian refugee agency.
"Today is test time for parties to the conflict to respect these area pauses to allow the UNRWA teams and other medical workers to reach children with these very precious two drops. It's a race against time," Ms Touma told Reuters.
Israel and Hamas, who have so far failed to conclude a deal that would end the war, said they would cooperate to allow the campaign to succeed.
WHO officials say at least 90% of the children need to be vaccinated twice with four weeks between doses for the campaign to succeed, but it faces huge challenges in Gaza, which has been largely destroyed by the war.
"Children continue to be exposed, it knows no borders, checkpoints or lines of fighting. Every child must be vaccinated in Gaza and Israel to curb the risks of this vicious disease spreading," said Ms Touma.
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Meanwhile, Israeli forces continued to battle Hamas-led militants in several areas across the Palestinian enclave. Residents said Israeli army troops blew up several houses in Rafah, near the border with Egypt, while tanks continued to operate in the northern Gaza City suburb of Zeitoun.
Israel has recovered the bodies of six hostages from a tunnel in southern Gaza where they were apparently killed not long before Israeli troops reached them, the military said.
The war was triggered after Hamas militants on 7 October stormed into southern Israel killing 1,200 people and taking more than 250 hostages by Israeli tallies.
Since then, at least 40,691 Palestinians have been killed and 94,060 injured in Gaza, the enclave's health ministry says.
