The UN's humanitarian chief, Sir John Holmes, visited Gaza on Thursday and said that he was shocked by "the systematic nature of the destruction".
Israel had prevented International aid agencies from entering Gaza since early November.
Vast amounts of building materials is needed immediately to rebuild hospitals, mosques, public buildings and homes.
He also said that the scale of destruction would have "disturbing" repercussions for the people of Gaza.
In one industrial area every building within a square kilometre had been levelled, by bulldozers and shells.
The full scale of the "systematic destruction" of Gaza is slowly emerging.
Who was it aimed at? Hamas or the innocent Palestinians?
Israel had prevented International aid agencies from entering Gaza since early November.
Vast amounts of building materials is needed immediately to rebuild hospitals, mosques, public buildings and homes.
He also said that the scale of destruction would have "disturbing" repercussions for the people of Gaza.
In one industrial area every building within a square kilometre had been levelled, by bulldozers and shells.
The full scale of the "systematic destruction" of Gaza is slowly emerging.
Who was it aimed at? Hamas or the innocent Palestinians?