Sir John Holmes, who visited Gaza on Thursday, said he was shocked by "the systematic nature of the destruction".
He said that the territory's economic activity had been set back by years.
The International Committee of the Red Cross's (ICRC) president Jakob Kellenberger said conditions in Gazan hospitals were the worst he had seen.
UN workers have been given access to Gaza. On Friday, Israel lifted a ban on international aid agencies entering the Palestinian territory.
The ban had been in place since early November when tensions mounted between Israel and Hamas....
[Michael Holmes of Oxfam said] A key problem facing them...is that the main crossing for the aid is 40km from where most of the relief is needed and is too small for the number of trucks that need to go through.
Future of Gaza
Mr Kellenberger told the BBC: "What I have seen in the hospital in Gaza was for me as far as a visit is concerned... it was my most shocking experience."
He added that the Israeli forces should not be using weapons such as phosphorus shells which made it difficult to avoid injuring or killing civilians.
Mr Holmes, the top UN official responsible for emergency relief and humanitarian affairs, said the scale of destruction would have "disturbing" repercussions for the people of Gaza.In an interview with the BBC's Today Programme, he described an industrial area where every building within a square kilometre had been levelled, by bulldozers and shells. He told of broken pipes pumping out raw sewage onto the streets. "I'm sure the Israelis would say that's because there were people there firing shells and rockets from there, or perhaps manufacturing them. "But the nature of that destruction means that any kind of private economic activity in Gaza is set back by years or decades," he said.