The United Nations, in collaboration with Palestinian health authorities, began to vaccinate 640,000 children in the Gaza Strip on Sunday, with Israel and Hamas agreeing to brief pauses in the conflict to allow the vaccination campaign.

The campaign began in areas of central Gaza, and will move to other areas in the coming days. Fighting will be paused for at least eight hours on three consecutive days, CNA News reported.

Rik Peeperkorn, head of the World Health Organization office for the West Bank and Gaza, said 2,180 workers in 295 teams would administer the oral vaccines to 640,000 children under 10 years of age at 392 sites. The vaccine is to be delivered in two doses about four weeks apart.

Sam Rose, senior deputy director of the UN Palestinian refugee agency UNRWA in Gaza, said the lack of access to aid, services, water and healthcare creates a situation under which polio has recently reemerged in Gaza "with a small number of cases that could spread very rapidly".

"It's really that perfect storm of conditions that created the environment in which polio can occur," he said in a video released on the UNRWA website.

Juliette Touma, communications director at UNRWA, told Reuters on Sunday that the vaccination campaign was massive and "one of the most complex in the world".

"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," Touma said.

"Children continue to be exposed. Every child must be vaccinated in Gaza and Israel to curb the risk of this vicious disease spreading."

WHO officials have said at least 90 percent 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 continuing conflict.

The Health Ministry in Gaza said on Sunday that at least 40,738 people have been killed in the conflict, now in its 11th month. The toll includes 47 deaths in the previous 24 hours.

Meanwhile, families of Israeli hostages have called for a nationwide general strike starting on Sunday night to force the government to reach a deal to secure the release of captives still held in Gaza.

The call came after the Israeli military found the bodies of six hostages in a tunnel in the southern Gaza city of Rafah, sparking outrage and anger among their families.

Xinhua and Agencies contributed to this story.