Hundreds of millions of dollars in humanitarian aid will be needed immediately to help Gaza's 1.4 million people and billions of dollars will be required to rebuild its shattered buildings and infrastructure, the U.N. humanitarian chief said Monday.

John Holmes said some neighborhoods have been almost totally destroyed, there are huge medical and food needs, sewage is flowing in some streets, and unexploded ordnance is posing a big problem.

While 100,000 people had their running water restored on Sunday, 400,000 still have no water, electricity is available for less than half the day, and 100,000 people are displaced from their homes, Holmes said.

"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..." Holmes told reporters at U.N. headquarters.

U.N. Secretary-General Ban Ki-moon said over the weekend that he was sending a U.N. team to assess the humanitarian needs and wanted a report in 10 days so the U.N. can issue an emergency appeal for funds.

Speaking to reporters at U.N. headquarters, Holmes said U.N. staff in Gaza are already trying "to find out as much as they can about how great the damage is and how great the needs are."

Asked to estimate the costs, Holmes said he couldn't give exact figures until the assessments are completed.

"I think on the purely humanitarian and early recovery side ... it will be hundreds of million of dollars," he said, "and no doubt the overall reconstruction costs will be numbered in billions of dollars, but I wouldn't want to put a figure on it beyond that."

He welcomed a $1 billion pledge to Gaza's reconstruction from Saudi Arabia.

Holmes stressed, however, that to successfully rebuild Gaza, the current "temporary and fragile cease-fire" must be transformed into a permanent and durable truce, with all border crossings opened to allow full access for humanitarian staff and to revive the economy.

As for initial assessments since the temporary cease-fires, Holmes said, "a total of 50 U.N. facilities, by our calculation, have been damaged during the hostilities so we have a huge amount of reconstruction of our own."

He said there is also "enormous strain on hospitals and medical facilities, not least because 21 medical facilities were damaged during the fighting."

John Ging, head of Gaza operations for the United Nations Relief and Works Agency, told reporters by videolink that about 20,000 people who had taken refuge at U.N. shelters were expected to stay in their own homes Monday night.

"This leaves us with well over 35,000 in our shelters tonight," he said. "These shelters are schools, and this will be an urgent priority for us to deal with so that we can resume our education program."