A senior UN official says massive assistance is needed for people who have had their homes destroyed by the Israeli army in Gaza.
The girl was walking with a group of older children when killed
Angry Palestinians mobbed a UN aid shipment and smashed the windows of UN vehicles, as Peter Hansen toured affected areas.
A three-year-old Palestinian girl has been shot dead - by Israeli soldiers, according to her relatives.
The Israeli army said it had no reports of shooting in the area at the time.
The girl was walking to a shop to buy sweets, and her family say she was hit in the head by Israeli gunfire.
"We were playing in the house when she told me she wanted some candy," said her 19-year-old brother Diyab Abu Zeid.
"The older kids in the neighbourhood were going to the shop so I let her go with them.
"There was no one in the street but the kids, not even other adults," he added.
There was also violence in the West Bank on Saturday, where a suicide bomber blew himself up at an army checkpoint, injuring one soldier, Israel Radio reported.
On Friday, Israeli forces pulled out of parts of Rafah after three days of fighting and occupation, in which more than 40 Palestinians were killed.
Rafah's residents have been angered by the scale of destruction left behind - dozens of homes were demolished and streets were torn up during the operation. Even the local zoo was left in ruins - the animals died or escaped.
The Israeli army says it had found a tunnel which was used for smuggling arms from Egypt.
"I think that the destruction is probably even worse than I've seen ... and is indeed completely, completely unacceptable," said Peter Hansen, head of the UN Relief and Works Agency, which provides support for Palestinian refugees.
Meanwhile a senior World Health Organisation official has expressed concern about the health services in Palestinian areas, saying they could be on the brink of collapse.
Dr David Nabarro said the current security crisis and restrictions on movement had left vulnerable people in the Gaza Strip and West Bank at greater risk of malnutrition and disease.
The offensive was launched after 13 Israeli soldiers were killed by Palestinian militants in the Rafah area last week.
The local leader of the radical Hamas movement died in an Israeli missile strike, Palestinian medical sources said.
Rafah's mortuary overflowed and many of the dead have had to be stored in freezers in different parts of town.