The UN said the tank fire and air attacks were preventing medical staff reaching hospitals and ambulances could not get to injured "because of continuous fire."
The World Food Programme has coordinated emergency food deliveries into Gaza in recent months but the Israeli army said there was plenty of food in Gaza warehouses and that the territory's Hamas rulers had halted distribution.
The Israeli government is adamant there is no humanitarian crisis in Gaza.
"The WFP stopped sending food in there because their warehouses are full to the top," military spokeswoman Major Avital Leibovitz told AFP
"The question is why Hamas is not moving the food around the territory. They say the roads are blocked. Why are the roads blocked for food but they can get around to fire rockets?"
Christine Van Nieuwenhuyse, the WFP representative for the Palestinian territories, told AFP however that the Gaza food warehouses were at less than half capacity.
She said food could not be distributed because it is "too dangerous" in the conflict or because warehouses were in military zones.
Gaza border crossings have been closed for two days and she said the WFP had asked the Israeli government to allow more trucks to go into Gaza.
"Tomorrow (Monday) we hope to send some in. But some roads have been destroyed and some of the Palestinian transporters are afraid to go to the border," Van Nieuwenhuyse said.
Source-AFP
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