Unwra warned said its food distribution operations would end on Thursday
The UN in the Gaza Strip says it will run out of food aid in two days unless Israel's blockade - which it describes as "shameful and unacceptable" - eases.
The UN refugee agency Unrwa, which distributes food to half of Gaza's 1,5m people, called the blockade "a physical as well as a mental punishment".
Israel is now allowing a limited amount of fuel across the border, but it is still blocking food deliveries.
It says it tightened sanctions because of rocket attacks by militants.
The Islamist group Hamas, which controls Gaza, said the rockets were a response to an Israeli raid that killed six gunmen on 4 November.
Gaza's only power plant was closed on Monday, after Israel stopped fuel deliveries.
Aid agencies estimate the new deliveries of fuel will run out within a day-and-a-half.
In a statement, Unrwa spokesman Christophen Gunness said food distribution operations would end on Thursday unless Israeli authorities allowed deliveries of wheat, luncheon meat, powdered milk and cooking oil without delay.
It is a further illustration of the barbarity of this inhuman blockade
Christopher Gunness Unrwa spokesman
"This is both a physical as well as a mental punishment of the population - of mothers and parents trying to feed their children - who are being forced to live hand to mouth," he said.
"It is a further illustration of the barbarity of this inhuman blockade."
"It is also shameful and unacceptable that the largest humanitarian actor in Gaza is being forced into yet another cycle of crisis management," Mr Gunness added.
Earlier, the UN described fuel shortages in the Gaza Strip as "real and serious".
Most of Gaza City was plunged into darkness on Monday evening after the Nusseirat power plant - the only one inside the territory - was forced to shut down because fuel shipments had been blocked by Israel for nearly a week.
Israel sealed the border after a wave of Palestinian
The facility depends on supplies of industrial fuel, mainly paid for by foreign donors, but transported through the Israeli-run Nahal Oz terminal.
A delivery of 800,000 litres of fuel was received on Tuesday, a European Commission spokeswoman said, and more is expected in the coming days.
The Nusseirat power plant provides about a quarter of the Gaza Strip's electricity and more than half the electricity used by the city itself. Most of the rest of the supply comes directly via power lines from Israel.
Meanwhile, Palestinian Authority President Mahmoud Abbas has used a speech on the fourth anniversary of the death of his predecessor, Yasser Arafat, to blame Hamas for another failed attempt at reconciliation with his Fatah movement.
Ban Ki-moon appealed for both sides to respect the truce
Hamas boycotted Egyptian-sponsored reconciliation talks that were to have begun in Cairo on Monday, saying security forces loyal to Mr Abbas had arrested hundred of its members in the West Bank.
"Now it's clear who is not serious... Arab countries should intercede and condemn Hamas," Mr Abbas told a demonstration in the West Bank town of Ramallah.
He also accused Hamas, which seized control of Gaza in June 2007, of putting its own interests before those of the Palestinian people.
"We will not call anybody a non-believer or traitor, as they do," he said.
"They must return to the fold of the homeland and think about the interests of the homeland. Regrettably, they lost that opportunity."
A Hamas spokesman said Mr Abbas had "not shown good intentions... towards a successful future for Palestinian national dialogue".
"We can't build a promising future for national unity on this harsh, contradictory speech which was full of hatred toward Hamas," Fawzi Barhoum told the Associated Press.
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