At least 23 people were killed and 73 others were injured after Israel struck a UNRWA school in a designated safe zone where displaced people are sheltering. Five UNRWA schools have been hit in the last 10 days, according to the UNRWA.

"UN facilities must be protected at all times. They must never be used for military or fighting purposes. No one is safe in Gaza, wherever they are. The people of #Gaza are children, women & men who have the right to live," the UNRWA said Tuesday.

In another strike on a safe zone in Mawasi Khan Yunis, at least 17 people were killed and 26 others were injured.

The Israel Defense Forces confirmed that it struck the school, alleging it struck terrorists who were operating in a UNRWA school.

-ABC News' Will Gretsky

Israel has carried out a strike on Nuseirat refugee camp, where internally displaced Palestinians have been told to shelter, for the second day in a row, according to Gaza Civil Defense.

This is the sixth school -- a designated safe zone -- to be targeted by Israeli Defense Forces' airstrikes in one week.

Israel has not confirmed whether Saturday's strike that killed 90 Palestinians killed two Hamas officials, including military chief Mohammed Deif, Israeli Prime Minister Benjamin Netanyahu said in a press conference.

"There is still no absolute certainty that the two have been eliminated, but I want to assure you that one way or another we will reach the entire top of Hamas," Netanyahu said.

Netanyahu said he was briefed about the type of weapons that would be used and the expected "collateral damage," as well as confirming Israel did not believe any hostages were held in the area, before giving the go ahead for the strike.

"Why should we risk something leaking out? Suppose something leaked, Deif and his deputy would go underground in a second. We update our American friends when necessary," Netanyahu said.

Asked about not telling the U.S. about the strike beforehand, Netanyahu said it was to avoid an information leak.

-ABC News' Anna Burd

The United Nations Human Rights Office has condemned the Israel Defense Forces' use of weapons in populated areas of Gaza, including humanitarian zones, hours after an attack killed 90 Palestinians.

"The latest attack and casualty followed right after another massive attack on the north, which lasted for a week, resulting in further destruction and casualties," the UN Human Rights Office said in a statement.

The U.N. said the IDF's use of weapons in densely populated areas "despite the overwhelming evidence that these means and methods have led to disproportionate harm to civilians and damage to civilian infrastructure, suggests a pattern of willful violation of the disregard of [International Humanitarian Law] principles of distinction, proportionality and precaution."

"The use of such weapons in an area to which IDF is ordering people to evacuate demonstrates a rampant disregard for the safety of civilians. Even if Palestinian armed group members were using the presence of civilians in these areas in an attempt to shield themselves from attack, which would violate IHL, this would not remove IDF's obligations to comply with these fundamental IHL principles of proportionality, distinction and precaution," the U.N. said.