Palestinian authorities in Gaza claimed that a United Nations school was hit by Israeli tank shells early Wednesday

Starting at around 4:30 a.m., several shells hit the compound of the Abu Hussein school, a few minutes apart, said the principal, Fayez Abu Dayeh. He said shells hit two classrooms and a bathroom.

In one of the classrooms, the front wall was blown out, leaving debris and bloodied clothing. Another strike tore a large round hole into the ceiling of a second floor class-room.

Four of the dead were killed just outside the school compound, two in their home and two who were struck in the street after returning from pre-dawn prayers, their relatives said.

"It's the responsibility of the world to tell us what we shall do with more than 200,000 people who are inside our schools, thinking that the U.N. flag will protect them," he said. "This incident today proves that no place is safe in Gaza."

Israel has vowed to stop the Hamas rocket and mortar fire that has reached increasingly deeper into its territory and to destroy a sophisticated network of Hamas military tunnels used for attacks in Israel.

For its part, Hamas has so far rejected cease-fire efforts unless its demands are met, including a lifting of a punishing blockade.