United Nations (AFP)

The United Nations said on Monday it had been forced to halt its humanitarian operations in Gaza due to a new Israeli evacuation order for the Deir al-Balah area, a senior UN official said.

"We are not operating today. As of this morning, we're not operating in Gaza," the official said, adding that since the start of the war the UN has sometimes had to "delay or take a pause."

"This is not a decision that we're saying we're stopping to operate, but practically we cannot operate," the official said.

According to the official, the UN "had relocated most of our personnel in our operations" to Deir al-Balah following a Rafah evacuation order several months ago.

"We need to find solutions," the UN official said. "And if it means that we need to anchor down for 24-48 hours and reset, we do that. But we're not leaving.

The evacuation order came as the UN prepares to begin on Saturday a campaign to vaccinate an estimated 640,000 children in Gaza, where the World Health Organization (WHO) said a 10-month-old baby had been paralysed by the type 2 polio virus, the first such case in the territory in 25 years.

"We're unable to deliver today with the conditions that we're in," said the senior UN official, speaking on condition of anonymity.

"Right now the challenge is to find a place where we can reset and effectively operate."