At least thirteen people have been killed in a tank shell attack on a United Nations school in Gaza, where hundreds of Palestinians had taken refuge from Israeli attacks.

Gaza health official Ashraf al-Kidra said another 90 Palestinians were wounded in the shelling early today.

Adnan Abu Hasna, a spokesman for a UN aid agency, said tank shells hit the Abu Hussein school in the Jabaliya refugee camp at about 4.30am.

An AP reporter who arrived later at the school saw a large round hole in the ceiling of one classroom and in one of the bathrooms. In another classroom, the strike had blown out the front wall.

The incident comes a day after Israel unleashed its heaviest air and artillery assault in the bloodiest day of the three-week Gaza war, destroying key symbols of Hamas control and shutting down the territory's only power plant.

The carnage began when several tank shells hit the school compound, a few minutes apart, said school 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 classroom.

The Israeli army had no immediate comment.