Rebuilding the Gaza Strip after Israel's three-week offensive will cost billions of dollars, the UN has warned.

Tens of thousands of Palestinians have been left homeless and 400,000 people still have no running water, it says.

The UN Secretary General, Ban Ki-moon, is currently visiting northern Gaza to see what assistance can be provided.

Ceasefires declared by Palestinian militant groups and Israel are holding, and Israeli troops are expected to complete their pull-out later.

Israeli political sources say the military aims to have withdrawn before Barack Obama's inauguration as the new president of the United States at 1700 GMT.

But analysts say big questions remain, such as who will police Gaza's southern border with Egypt and how much power Hamas still has.

Israeli Prime Minister Ehud Olmert has said he wants troops to leave Gaza "as quickly as possible" and some have already left. Hamas has said it will hold fire until Sunday to give Israel time to withdraw.

Meanwhile, an Israeli man was shot and seriously wounded while driving near the Kochav Hashachar settlement in the West Bank.

A previously unknown group calling itself the "al-Bashair Army" told the Palestinian news agency, Maan, it had carried out the attack.

Rubble

At a news conference in New York, UN Under-Secretary General for Humanitarian Affairs John Holmes said hundreds of millions of dollars of aid would be needed immediately to help Gaza's 1.4 million people in the aftermath of the offensive, which ended on Saturday.

Mr Holmes said some neighbourhoods had been almost totally destroyed and many homes reduced to rubble.

Sewage was flowing in the streets, there were huge medical and food needs, and unexploded ordnance was posing a big problem, he said.

While 100,000 people had their water supply restored on Sunday, 400,000 still have no water, he said.

Electricity is available for less than 12 hours a day, and 100,000 people had been displaced, he added.

A total of 50 UN facilities and 21 medical facilities were damaged, he added.

"It may not be very clear who actually won this conflict, if such a concept means anything in Gaza, but I think it's pretty clear who lost and that was the civilian population of Gaza, and to a much lesser extent the civilian population of southern Israel," Mr Holmes told reporters at UN headquarters.

When asked to estimate the costs, Mr Holmes said he could not give exact figures until UN teams in Gaza had completed their assessments, but the likely figure was hundreds of millions, and the overall reconstruction costs would run into billions.

Separately, the Palestinian Central Bureau of Statistics said on Monday that 4,100 homes were totally destroyed and 17,000 others damaged during the conflict.

About 1,500 factories and workshops, 20 mosques, 31 security installations and 10 water or sewage pipes were also damaged, it added.

The bureau estimated that the overall physical damage so far amounted to about $1.9bn (£1.4bn), including about $200m (£140m) of damage to infrastructure.

The Israeli Social Welfare Minister, Isaac Herzog, told the BBC the problem was how to rebuild Gaza without facilitating further attacks on Israel by militants.

There was a risk during reconstruction that buildings would be turned into missile launch sites, he said.

UN Secretary General Ban Ki-moon earlier met Israel's prime minister in Jerusalem before entering northern Gaza via the Erez Crossing, to inspect the damage and see what assistance could be given.

He will visit three UN facilities which were heavily damaged during the conflict, including a school where at least 40 people taking shelter were killed when what the Israeli military says was a "stray mortar" fired by its troops landed outside.

UN spokesman Ahmed Fawzi said Mr Ban was keen to express solidarity with the people of Gaza and for the UN staff, who he said had continued heroically to provide assistance under such difficult circumstances despite some of them being killed.

He will later visit the southern Israeli town of Sderot to see the damage caused by rocket attacks from Gaza.

The BBC's Aleem Maqbool in Gaza City says Palestinians are meanwhile continuing to search through the rubble of their homes to try to find the bodies of those killed in the conflict.

Many are angry and feel that the world did not do enough when it mattered to stop the violence, our correspondent says.

'Horrific injuries'

Israel called a ceasefire on Saturday, saying it had met its war aims. Hamas later declared its own truce, with one of its leaders claiming a "great victory" over Israel.

Palestinian medical sources say at least 1,300 Palestinians were killed, nearly a third of them children, and 5,500 injured during the conflict. Thirteen Israelis, including three civilians, were killed.

The director of operations for UN aid agency in Gaza, Unrwa, told the BBC that the weapons used by the Israeli military had caused "horrific" injuries to children.

"These are not scratches or bullet wounds, these are kids who are hit by shrapnel in most instances," John Ging said.

Palestinian militant groups in Gaza meanwhile said 112 of their fighters and 180 Hamas paramilitary policemen were killed, according to the Reuters news agency.

Arab foreign ministers meeting on the sidelines of an economic summit in Kuwait City have meanwhile failed to agree on a unified statement on Gaza because "some are entrenched in their positions", Iraqi Foreign Minister Hoshyar Zebari has said.