UNITED NATIONS - Secretary-General Ban Ki-moon condemned the attack on a U.N. school in Gaza that killed at least 16 people early Wednesday as "outrageous" and "unjustifiable," and demanded an immediate humanitarian cease-fire in the war between Israel and Hamas.

"Nothing is more shameful than attacking sleeping children," Ban said on his arrival in San Jose, Costa Rica. "I condemn this attack in the strongest possible terms."

Ban said "all available evidence points to Israeli artillery as the cause" of the pre-dawn attack, and he pointedly noted that Israeli military authorities had received the coordinates of the school from the United Nations 17 times, including on Tuesday night.

At U.N. headquarters in New York, Deputy Secretary-General Jan Eliasson expressed "shock and dismay" at the attack that also injured more than 100. He stressed that more than 3,000 civilians had sought shelter at the elementary girls' school in the Jebaliya refugee camp.

Israeli's military said it fired back after soldiers were targeted by mortar rounds launched from the vicinity of the school.

John Ging, the director of U.N. humanitarian operations, said it was the fifth attack on a United Nations school sheltering civilians since fighting between Israel and Hamas began on July 8.

Ban said almost 1,300 people have been killed in Gaza since then, almost 6,000 have been wounded, and the United Nations has taken in 140,000 people who have fled the fighting at its facilities in Gaza, including many schools.

The U.N. Security Council scheduled a briefing Thursday morning on the dire humanitarian situation in Gaza at the request of Jordan, the Arab representative on the council.

Eliasson asked the world to remember the human beings at the center of the war.

He took a copy of the U.N. Charter out of his pocket and read the first few words, "We the peoples..." to stress that "this organization is about people in the end."

"We've just seen this morning a horrifying example of the price paid," Eliasson said. "When you get that vicious circle of violence, first of all you get increased suffering, and secondly you get hatred and mistrust that is there as a poison in the societies and makes a peaceful future less probable."

Eliasson and Ban urged both sides to agree to an immediate, unconditional humanitarian cease-fire and talks to resolve outstanding issues peacefully.