DEIR AL-BALAH, Gaza Strip — Palestinian health authorities and United Nations agencies on Sunday began a large-scale campaign of vaccinations against polio in the Gaza Strip, hoping to prevent an outbreak in the territory that has been ravaged by the Israel-Hamas war.

Authorities plan to vaccinate children in central Gaza until Wednesday before moving on to the more devastated northern and southern parts of the strip. The campaign began with a small number of vaccinations on Saturday and aims to reach about 640,000 children.

The World Health Organization has said Israel agreed to limited pauses in the fighting to facilitate the campaign. There were initial reports of Israeli strikes in central Gaza early Sunday, but it was not immediately known if anyone was killed or wounded. The pause ended Sunday afternoon, according to a schedule released by Israel.

Hospitals in Deir al-Balah and Nuseirat confirmed that the campaign had begun. Israel has said the vaccination program will continue through Sept. 9 and last eight hours a day.

Gaza recently reported its first polio case in 25 years — a 10-month-old boy, now paralyzed in the leg. The World Health Organization says the presence of a paralysis case indicates there could be hundreds more who have been infected but aren’t showing symptoms.

Most people who have polio do not experience symptoms, and those who do usually recover in a week or so. But there is no cure, and when polio causes paralysis, it is usually permanent. If the paralysis affects breathing muscles, the disease can be fatal.

The vaccination campaign faces challenges, from ongoing fighting to devastated roads and hospitals shut down by the war. Around 90% of Gaza’s population of 2.3 million people have been displaced within the besieged territory, with hundreds of thousands crammed into squalid tent camps.

Health officials have expressed alarm about disease outbreaks as uncollected garbage has piled up and the bombing of critical infrastructure has sent putrid water flowing through the streets. Polio is spread through fecal matter. Widespread hunger has left people even more vulnerable to illness.

“We escaped death with our children, and fled from place to place for the sake of our children, and now we have these diseases,” said Wafaa Obaid, who brought her three children to the Al-Aqsa Martyrs Hospital in Deir al-Balah to get the vaccinations.

Ammar Ammar, a spokesperson for the U.N. children’s agency, said it hopes both parties adhere to a temporary truce in designated areas to enable families to reach health facilities.

“This is a first step,” he told The Associated Press. “But there is no alternative to a cease-fire because it’s not only polio that threatens children in Gaza, but also other factors, including malnutrition and the inhuman conditions they are living in.”

The vaccinations will be administered at roughly 160 sites across the territory, including medical centers and schools. Children under 10 will receive two drops of oral polio vaccine in two rounds, the second to be administered four weeks after the first.

Israel allowed around 1.3 million doses to be brought into the territory last month, which are now being held in refrigerated storage in a warehouse in Deir al-Balah. Another shipment of 400,000 doses is set to be delivered to Gaza soon.

The polio virus that triggered this latest outbreak is a mutated virus from an oral polio vaccine. The oral polio vaccine contains weakened live virus and in very rare cases, that virus is shed by those who are vaccinated and can evolve into a new form capable of starting new epidemics.

The war in Gaza began when Hamas-led militants stormed into Israel on Oct. 7, killing some 1,200 people, mostly civilians, and abducting around 250 hostages. Around 100 remain in captivity, about a third of whom are believed to be dead.

Israel’s retaliatory offensive has killed over 40,000 Palestinians, according to the Gaza Health Ministry, which does not say whether those killed were fighters or civilians. The war has caused vast destruction across the territory, with entire neighborhoods wiped out and critical infrastructure heavily damaged.

The United States, Egypt and Qatar have spent months trying to broker a cease-fire and the release of the remaining hostages, but the talks have repeatedly stalled and a number of sticking points remain.