"The UN is suspending its aid operations in Gaza until we can get safety and security guarantees for our staff," spokesman Chris Gunness said. "We've been coordinating with them (Israeli forces) and yet our staff continue to be hit and killed."
For a second straight day, Israel said it suspended its Gaza military operation for three hours to allow in humanitarian supplies. Shortly before the pause took effect, however, the UN said one of its aid trucks came under Israeli fire, killing the driver.
UN spokesman Adnan Abu Hasna said the UN coordinated the delivery with Israel, and the vehicle was marked with a UN flag and insignia when it was shot in northern Gaza. The Israeli army said it was investigating.
The UN has already demanded an investigation into Israel's shelling of a UN school in Gaza that killed nearly 40 people earlier this week. Israel and residents said militants were operating in the area at the time.
The UN provides food aid to around 750,000 Gaza residents, and runs dozens of schools and clinics throughout the territory. They have some 9,000 locally-employed staffers inside Gaza, and a small team of international staffers who work there.
At least one Palestinian has been killed after a UN relief agency convoy came under fire from Israeli forces in the Gaza Strip, officials say.
The attack took place on Thursday as the lorries travelled to the Erez crossing to pick up supplies that were to have been allowed in during a three-hour ceasefire.
Thursday's pause in the 13-day Israeli offensive to allow humanitarian aid into the strip was scheduled to last from 1pm (11:00 GMT) to 4pm (14:00 GMT).
The Israeli military said that it was checking reports of the incident.
John Ging, the head of the UN relief agency in Gaza, said that the casualties were Palestinian civilian contractors contracted to bring supplies from the crossing points.
"They were co-ordinating their movements with the Israelis, as they always do, only to find themselves being fired at from the ground troops," he told Al Jazeera.
"It has resulted tragically in the death of one and the injury of two others."
Wednesday's three-hour ceasefire allowed beleaguered Gazans and aid workers to recover dead bodies, treat the wounded, and gather much-needed supplies in and around Gaza City.
Earlier on Thursday, thousands of Palestinians fled their homes in the southern Gaza Strip as Israeli forces bombarded Rafah after dropping leaflets to warn local residents about an impending blitz.