Ban Ki-moon, the UN secretary-general, crossed from Israel into Gaza on Tuesday for what a spokesman described as a visit to express solidarity with Palestinian suffering.
Estimates for the rebuilding of Gaza's devastated infrastructure have been put at billions of dollars.
Dire situation
John Holmes, the UN humanitarian chief, says hundreds of millions of dollars in emergency aid supplies will be needed for the people of Gaza.
Although 100,000 people had running water restored in their homes as of Sunday, 400,000 were still without it, Holmes said.
Electricity in Gaza is available for less than half the day and about 100,000 people have been displaced by the war.
Despite the three-week Israeli onslaught that killed more than 1,300 Palestinians and destroyed thousands of buildings, Hamas and other Palestinian factions claimed victory in the fighting.
Israel had said the aim of its operations in Gaza was to cripple Hamas's ability to launch rockets into the south of the country.
But a masked man calling himself Abu Obeida and claiming to be a spokesman for Ezzedine al-Qassam Brigades, Hamas's armed wing, said the group's rocket-launching capacity had not been diminished, and threatened to renew fighting if Israeli forces did not withdraw.
"They [Israel] say they weakened Hamas. We assure you that what we have lost in this war is nothing compared to what we [still] have," he said in a televised news conference on Monday.
Abu Obeida vowed that Hamas would replenish its arsenal of rockets and other weapons, in defiance of any Israeli or international efforts to cut off smuggling routes.
"Do whatever you want, bringing in and manufacturing the holy weapons is our mission, and we know how to acquire weapons," he said.
Disease fears
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At least 1,300 people killed, including more than 400 children and more than 100 women
At least 5,300 Palestinians injured, including nearly 1,900 children and 800 women
At least 100,000 people forced from their homes
At least 13 Israelis killed, including three civilians
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Meanwhile, scores of bodies have been discovered in the rubble of destroyed buildings since the fighting was halted.
Abed Sharafi, an ambulance driver, said on Monday that he had helped pull out the bodies of 15 children and women from under their house.
"They were so badly decomposed that we couldn't distinguish boys from girls. Some had been there for 15 days," he said.
Al Jazeera's Ayman Mohyeldin, reporting from Gaza City, said the World Health Organisation was warning of an outbreak of disease with bodies now several weeks old and sewage flowing over many areas because of the destruction to infrastructure.
The deposed Hamas-led government in Gaza estimates that more than 5,000 buildings were completely destroyed and 20,000 damaged or partially destroyed in the fighting.