"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.

The Israeli military had told people Sunday to "evacuate immediately."

"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.

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

Israel's war on Gaza has killed at least 40,435 people in Gaza, mostly women and children.