The spokesman said this was because Israel closed border checkpoints leading into the Palestinian enclave on December 5 due to rocket attacks across the border.
"On Saturday, the next working day, aid will not be distributed because today the humanitarian convoy was not allowed in and our warehouses are empty," he said.
"Due to the ongoing crisis with irregular border access and the lack of wheat flour in Gaza, UNRWA has exhausted all stocks of flour in its warehouses," the UN Relief and Works Agency (UNRWA) said in a statement. "Food distribution for both emergency and regular programs will be suspended from Thursday December 18 until further notice.
The Israeli blockade has halted the delivery of food, fuel and other basic needs to the impoverished territory, where about 750,000 people depend on assistance. About 20,000 Palestinians receive food through the UN every day.
A similar stoppage of humanitarian aid occurred on November 13 for the same reason.
The Israeli government said that it would allow a certain amount of aid into the enclave on Thursday, but then decided not to, citing existing threats from Palestinian militants. Israel blames Hamas for the humanitarian crisis in Gaza.
"If someone is looking for anyone to blame for the situation in Gaza, then the addressee is known: it is Hamas. Hamas is responsible for everything that is going on there," Israeli Foreign Minister Tzipi Livni said during a recent meeting with UN Secretary General Ban Ki-moon in New York.
"Gaza remains a source of terrorism and Hamas is doing nothing to even give the slightest legitimacy to its regime and actions," she said.
Israeli planes fired rockets into the Gaza Strip overnight, killing one person and injuring three others, a Palestinian radio said on Thursday.
The Israeli airstrike came in response to ongoing rocket attacks by Gaza-based militants.