More than 700 Palestinians have died in 16 days of violence between the Israeli military and Hamas militants.
Palestinians say residents of two villages in southern Gaza have been trapped by days of shelling from tanks, and medics have been unable to evacuate the wounded.
The UN said more than 140,000 people have been displaced, and condemned fresh attacks by the Israeli military on UN schools being used as shelters.
Efforts to reach a peace deal intensified, but a deal between Israel and Hamas fighters remains elusive.
Medical crews have not risked approaching victims in the southern villages of Khuzaa and Abassan, Palestinian officials said.