United Nations Secretary General Ban Ki-moon has condemned Israel’s attack on an UN-administered school that killed 15 people.

In a statement on Wednesday, Ban said the attack on the UN school, which was acting as a shelter for displaced civilians, was “cruel” and “senseless.” He reiterated his call for an immediate ceasefire.

“Nothing is more shameful than attacking sleeping children,’’ Ban said. “I condemn this attack in the strongest possible terms. It is outrageous. It is unjustifiable. And it demands accountability and justice.”

Ban said he wanted to make it clear that the precise location of Jabalia Elementary Girls School had been communicated to the Israeli military authorities 17 times - as recently ?as a few hours before the attack.

More than 1,300 people have been killed in the offensive, now in its fourth week. The United Nations is hosting 140,000 displaced persons in UN facilities, including schools.

UN Deputy Secretary General Jan Eliasson also said he was “shocked” and “appalled” by the attacks.

UN Humanitarian Operations Director John Ging said the school had been targeted five times since the conflict began.