John Ging, director of the main U.N. aid agency in Gaza, said there was plenty of food in warehouses but no way of getting it safely to the people in need.
"No fuel means that vital humanitarian aid and public services are grinding to a halt," Ging said.
Gaza depends on Israel for all its fuel but the Israelis reduced supplies in an effort to pressure Hamas to stop firing rockets across the border. Then, the flow was turned off completely after militants attacked an oil terminal on the Gaza border and killed two Israeli civilians.
The fuel blockade means pumps are off, causing water shortages and sewage problems, and the vaccination stocks at Gaza's main hospital were spoiled after it had to turn off its refrigerators, officials said.