UPDATE — 3:55 a.m. ET: A U.N. agency now says 19 people were killed and 126 injured after shells hit a school in northern Gaza:

GAZA CITY, Gaza Strip (AP) — A Palestinian health official says 13 people were killed after tank shells hit a U.N. school in Gaza where hundreds of Palestinians had taken refuge from Israeli attacks.

Gaza health official Ashraf al-Kidra said another 40 Palestinians were wounded in the shelling early Wednesday.

With displaced Palestinians still inside a youth walks by the debris in a classroom at the Abu Hussein U.N. school in Jebaliya refugee camp, northern Gaza Strip, hit by an Israeli strike earlier, on Wednesday, July 30, 2014. (AP Photo/Lefteris Pitarakis)

Adnan Abu Hasna, a spokesman for a U.N. aid agency, says tank shells hit the Abu Hussein U.N. school in the Jabaliya refugee camp around 4:30 a.m.

A displaced Palestinian boy, still inside a classroom, eats a snack at the Abu Hussein U.N. school in Jebaliya refugee camp, northern Gaza Strip, hit by an Israeli strike earlier, on Wednesday, July 30, 2014. (AP Photo/Lefteris Pitarakis)

An AP reporter who arrived later at the school saw what appeared to be tank shells in a bathroom and two classrooms.

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.