UN Secretary General Kofi Annan has demanded that Israel take urgent action to prevent a humanitarian disaster in the Gaza Strip.
Mr Annan is concerned about the effects on ordinary Palestinians
Israeli jets continued to pound Gaza targets on Sunday in operations aimed at securing the release of a soldier captured by Palestinian militants.
A Palestinian rocket hit the Israeli town of Sderot, injuring one man.
Mr Annan called on Israel to restore supplies of food and fuel and to repair a power plant hit in an air strike.
Mr Annan urged Israel to lift restrictions on the movement of basic goods such as foodstuffs into Gaza.
Israel has rejected a Hamas call for a ceasefire
He said UN agencies must be allowed to work in the region.
Israel has rejected a call by the head of the Hamas-led Palestinian government for a ceasefire.
Mr Annan's statement was his second in as many days about the situation in Gaza.
The BBC's Richard Galpin at the UN in New York says Mr Annan is clearly becoming increasingly alarmed by what is happening and is becoming increasingly blunt in his statements.
Mr Annan said the strike on the region's only power station had affected hospitals, water and sanitation plants, as well as food production.
In a separate statement, UN agencies including the World Health Organization, Unicef and the World Food Programme said Gaza was on the brink of a public health disaster.
They said there were water shortages and the situation at the sewage plants was now critical.
The WHO said hospitals and health centres - which are having to use their own generators for electricity - have at most two weeks' supply of fuel.
Earlier, Israel rejected a call by the head of the Hamas-led Palestinian government, Ismail Haniya, for all parties to restore calm through a mutual cessation of hostilities.
Officials in the Israeli PM's office said there would be no truce until the captured Israeli soldier, Cpl Gilad Shalit, was freed.
Hamas has confirmed that Cpl Shalit, 19, is alive and is being treated well and humanely.
Israel on Saturday said its troops had left their positions in northern Gaza.
But air strikes continued on Sunday.
Israel bombed a bridge in northern Gaza, saying it wanted to stop militants transporting rockets to launch sites.
Israeli aircraft also carried out early morning raids near the Karni commercial crossing with Gaza, injuring at least three militants.
Meanwhile, the Israeli army said Palestinian militants had fired a rocket from the north of Gaza into the Israeli town of Siderot, injuring one Israeli.
Dozens of Palestinians and an Israeli have died since Israel launched an offensive on 28 June to secure the release of Cpl Shalit.
Most of the Palestinians who have been killed were militants.
But Palestinian sources said a six-year-old girl, her 20-year-old brother and their mother died in an air strike on a house just east of Gaza City on Saturday.
Witnesses said an Israeli missile hit the house. Israel said its inquiry was continuing but it did not believe it was responsible for the incident.
Israeli forces remain in the south of the territory, as well as east of Gaza City.
Israel's incursion into Gaza is its biggest military operation there since it ended its 38-year occupation nine months ago.