The world body will temporarily remove about a third of its approximately 100 international staffers working in Gaza, UN Secretary-General spokesman Stéphane Dujarric said.
He pointed to the increased danger after Israel relaunched its military campaign last week with bombardment that has since killed hundreds of Palestinians.
Israel has also cut off all food, medicine, aid and other supplies to Gaza's population for the past three weeks.
Dujarric's statement was the UN's first to point the finger at Israel in the March 19 explosion at the UN guesthouse in central Gaza. He said that "based on the information currently available," the strikes on the site "were caused by an Israeli tank".
The Israeli military repeated its denial that it was responsible for the strike, which took place a day after Israel shattered Gaza's two-month-old ceasefire with a surprise bombardment across the Gaza Strip.
Dujarric said the UN "has taken the difficult decision to reduce the organisation's footprint in Gaza, even as humanitarian needs soar".
He says the UN "is not leaving Gaza," pointing out that it still has about 13,000 national staff in Gaza, mainly working for UNRWA, the UN agency for Palestinian refugees.
New Israeli strikes over the past 24 hours killed more than 60 Palestinians around Gaza, the territory's Health Ministry said. The ministry's count does not distinguish between civilians and militants.
The ministry on Monday put out a list of the names of more than 15,000 children, 17 and under, killed by Israel's campaign since it began more than 17 months ago.
The list included nearly 5000 children under the age of six who had been killed, including 876 babies.
Israel says it restarted its bombardment and cut off food to Gaza to force Hamas to accept new terms for the ceasefire and release more hostages.
It says it targets Hamas members and positions, blaming the group for civilian deaths because it operates among the population.
Air raid sirens and explosions were heard over Jerusalem, Tel Aviv, and central Israel on Monday evening as the military said it intercepted a missile launched from Yemen.
Yemen's Houthi rebels have fired a handful of long-range missiles at Israel in the days since it resumed the war in Gaza.
Two rockets from the Gaza Strip were also intercepted after crossing into Israeli territory earlier Monday evening, setting off sirens, the Israeli military said. There were no immediate reports of injuries.
Al-Quds Brigades, the military arm of the Palestinian Islamic Jihad militant group, claimed responsibility.
The International Committee of the Red Cross said its office in the southern city of Rafah in the Gaza Strip was damaged by an explosive projectile on Monday. No staff were hurt but the damage has a direct impact on its ability to operate.
The Red Cross also said that on Sunday, contact was lost with emergency medical technicians from the Palestine Red Crescent Society and their whereabouts remain unknown. Last week, humanitarian workers in Gaza were killed and injured, it said.
On Sunday, Israel struck the surgery ward at southern Gaza's biggest hospital, killing two people and wounding others, many of whom were already injured by earlier strikes.
One of those killed at Khan Younis' Nasser Hospital was a teenage boy recovering from surgery.
The other was a Hamas official that Israel says was the target of the strike, Ismail Barhoum. Hamas said Barhoum was undergoing treatment at the time. The Israeli military denied that, saying he oversaw Hamas' finances in Gaza, including transferring money to its armed wing, and was working out of the hospital.
The strike last week on the UN compound outside Deir al-Balah killed a 51-year-old staffer, Marin Valev Marinov. He worked with the UN Office for Project Services, which carries out infrastructure and development projects around the world.
Olga Cherevko, spokesperson for the UN 's humanitarian office, known as OCHA, said the UN and its partners have already suspended a number of activities, many in education, protection and water and sanitation services. The reason, she was, was safety concerns and the impact of Israeli evacuation orders.
Israel's campaign in Gaza has killed more than 50,000 Palestinians and wounded more than 113,000, according to the Health Ministry.
Nearly 90 per cent of the population of some 2,3 million have been driven from their homes. Israel launched the campaign vowing to destroy Hamas after its October 7, 2023 attack on southern Israel, in which militants killed some 1200 people and kidnapped 250 others.
