Valerie Amos told the Security Council that the world has watched "in horror the desperation of children and civilians that have come under attack" in Gaza.
Over 80 per cent of the more than 1,300 Palestinians killed were civilians, including 251 children, she said by videoconference from Trinidad and Tobago.
"The reality of Gaza today is that no place is safe," Amos said, citing attacks on over 103 UN facilities, including one on a school yesterday that killed 19 people and injured more than 100.
That is four times higher than the peak number of displaced people in the last Gaza conflict in 2008-09 and there are new arrivals every day, he said.
Also Read
"Conditions are increasingly dire in the shelters," he said. "There is no water for hygiene, very few showers, and latrines are totally inadequate. Disease outbreak is beginning with skin infections, scabies and others."
Krahenbuhl said UNRWA is concerned at reports of new instructions from the Israeli military to evacuate the Zeitoun area of Gaza City.