'Heart-wrenching split'
At least 649 Palestinians and 31 Israelis have been killed in the past 15 days of fighting, officials say.
A foreign worker in southern Israel was also killed by a rocket fired from Gaza on Wednesday, police said.
The UN says about 74% of those killed in Gaza are civilians, with medical clinics among the facilities hit by air strikes.
"Civilians in Gaza have no safe place to go as 44% of the land has been declared a 'no-go zone' by the Israeli army," the assistant secretary-general at the UN Office for the Coordination of Humanitarian Affairs said.
"Families are taking the heart-wrenching decision to split to different locations - mother and son to one; father and daughter to another - hoping to maximise the chance one part of the family survives."
State of panic
There was heavy fighting in the town of Khan Younis in the southern Gaza Strip on Wednesday.