The UN's humanitarian chief has told the BBC the situation in Gaza after a three-week Israeli offensive against Hamas was worse than he anticipated.
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.
Staff from a number of international non-governmental organisations (NGOs) were allowed to cross into Gaza on Friday morning.
Michael Bailey from Oxfam said: "We are extremely relieved to be able to join our local colleagues inside Gaza and to be able to provide them with some support and relief from the relentless work they have been doing."
CONFLICT IN FIGURES
More than 1,300 Palestinians killed
Thirteen Israelis killed
More than 4,000 buildings destroyed in Gaza, more than 20,000 severely damaged
50,000 Gazans homeless and 400,000 without running water
He described the task ahead as "enormous", with vast amounts of building materials alone needed immediately to help rebuild hospitals, mosques, public buildings and homes.
A key problem facing them, he said, 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.
He also urged Israel to end its policy of restricting the amount of cash Gazans can have access to, saying people in Gaza had run up "phenomenal debt" over the last few weeks, trying to buy goods that are in increasingly short supply.
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.
FROM THE TODAY PROGRAMME
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.
"That's very disturbing for the future of Gaza, for the future of the people of Gaza, who are forced to fall back on the public sector and indeed on Hamas, who control the public sector."
A humanitarian appeal was launched by a number of UK charities on Thursday to raise money for aid relief in Gaza.
War crimes claims
Israeli PM Ehud Olmert is reported to have placed his justice minister in charge of defending Israel against any accusations of war crimes.
Disasters Emergency Committee Gaza humanitarian appeal:
Launched by UK charities on 22 January to raise money for Gaza aid relief and reconstruction
Participants: Action Aid, British Red Cross, Cafod, Care International, Christian Aid, Concern Worldwide, Help the Aged, Islamic Relief, Merlin, Oxfam, Save the Children, Tearfund, World Vision
Tel: 0370 60 60 900 or go to DEC website
Daniel Friedman will lead an inter-ministerial team to co-ordinate a legal defence for Israeli civilians and the military, a government source was quoted by AFP as saying.
Richard Falk - the UN special rapporteur for human rights in the Palestinian territories - has said there was "a prima facie case" that Israel gravely breached the Geneva Conventions during its 22-day campaign.
Israel responded by saying that Mr Falk's "bias against Israel was well known".
Israel said it launched its offensive to stop cross-border rocket attacks by militants in Gaza against its civilians.
The intense fighting ended on Sunday, with both sides declaring a ceasefire.Palestinian medical officials said about 1,300 Palestinians were killed and thousands more were injured. Thirteen Israelis died during the conflict.