The U.N.'s secretary-general says today's apparent Israeli attack on a U.N. school in Gaza, killing at least 16 people, was ``outrageous'' and ``unjustifiable.'' And Ban Ki-moon is demanding an immediate humanitarian cease-fire in the war between Israel and Hamas.

Speaking today in Costa Rica, Ban said there's nothing more ``shameful'' than ``attacking sleeping children.'' He says ``all available evidence'' indicates Israeli artillery was involved in the pre-dawn attack.

He noted that Israeli military authorities had received the coordinates of the school from the United Nations 17 times, including last night.

Israel says it fired back after soldiers were targeted by mortar rounds from the vicinity of the school.

The head of U.N. humanitarian operations says it was the fifth attack on a United Nations school sheltering civilians since fighting between Israel and Hamas began on July 8th.