Israel says militants drew attack to school

GAZA CITY, Gaza (CNN) - The Israeli military said Hamas militants were firing mortars from a U.N. school being used as a shelter by hundreds of civilians Tuesday - prompting a retaliatory artillery strike that killed at least 40 people and wounded dozens more.

"We face a very delicate situation where the Hamas is using the citizens of Gaza as a protective vest," IDF spokesman Brig. General Avi Benayahu said after the incident.

Meanwhile, the United Nations was calling for an investigation of the strike at the northern Gaza school, which came a day after an airstrike hit an elementary school in Gaza City, killing three.

"The innocent people of Gaza want and are entitled to accountability," said United Nations Relief and Works Agency director John Ging.