The UN says rebuilding the Gaza Strip will cost billions of dollars
UN humanitarian chief John Holmes has urged Israel to fully open all crossings with Gaza to allow a free flow of goods.
Mr Holmes said that unless Israel allowed building materials into Gaza, no reconstruction could begin there.
The UN says rebuilding Gaza after Israel's three-week offensive against Hamas militants will cost billions of dollars.
Early on Wednesday, Israel said it had completed its troop pull-out from Gaza.
"The last soldier left the Gaza Strip this morning," an Israeli military spokesman told journalists.
"However the army remains deployed all around the Gaza Strip to meet any eventuality."
Palestinian medical sources in Gaza say at least 1,300 Palestinians were killed, nearly a third of them children, and 5,500 injured in the Israeli operation, which began on 27 December.
Thirteen Israelis, including three civilians, have been killed, the Israeli army says.
At a news conference in New York on Tuesday, Mr Holmes said it was "absolutely critical" that building materials - like cement and pipes - were allowed into Gaza.
"Otherwise, the reconstruction effort won't get off first base," he said.
The recent truce between Israel and Hamas militants does not include any deal on the opening of the crossings, which are tightly controlled by Israel.
So far Israel has been allowing only basic humanitarian supplies - like food and medicine - into Gaza.
Mr Holmes, who is expected to visit Israel on Wednesday, also stressed that he would be pushing Israel to allow international aid organisation into Gaza.
"In theory, they have permission. In practice, it's proving very difficult to get into Gaza."
EU foreign ministers are due to meet Israeli representatives for talks later on Wednesday.
Ban Ki-moon: 'I have seen heart-breaking scenes'
On Tuesday UN Secretary-General Ban Ki-moon said he was appalled by Israeli attacks on a UN compound in Gaza after seeing the destruction for himself.
Mr Ban said that those responsible should be held accountable and demanded a "full investigation".
He later visited the southern Israeli town of Sderot, which has been one of the main targets of Palestinian rocket attacks in recent years.
Mr Ban described the rockets as indiscriminate weapons and said the attacks by Hamas were violations of basic humanitarian law.