Israeli shelling around 5:30 am on Wednesday killed 20 people at a UN school being used as a shelter for the displaced persons, medics said.
A UN official confirmed the incident in Jabalia refugee camp, but gave an initial toll of between 13 and 15 dead.
Earlier in the northern Gaza Strip, Israeli shelling killed an 11-year-old handicapped girl, emergency services spokesman Ashraf al-Qudra said, with a subsequent shelling in the centre of the small coastal territory killing a 16-year-old girl.
Shortly afterwards a strike in the southern city of Khan Yunis killed nine members of a family, Qudra said.
A middle-aged man was killed in the early morning attack in the southern city of Rafah.
Gaza death toll in last 24 hours has reached to 162, while the total Palestinian death toll from Israeli offensive has reached to at least 1,262, according to Palestinian sources.
Palestinian militants rocket fire has killed 53 Israeli soldiers and three civilians inside Israel.