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“Children killed in their sleep; this is an affront to all of us, a source of universal shame. Today the world stands disgraced," he said.
More than 200,000 displaced Palestinians, many heeding Israeli evacuation orders, have taken shelter in U.N. schools and buildings across the besieged territory.
The U.N. said it had communicated to the Israeli military 17 times before Wednesday’s attack — including once just hours before the fatal shelling — that thousands of internally displaced Palestinians had sought refuge in the school.
The White House condemned the school shelling, but did not assign responsibility for the attack to either Israelis or Palestinians.
Meanwhile, the Pentagon confirmed that the U.S. has agreed to replenish Israel's dwindling supplies of mortar rounds and grenade launchers.
Wednesday's shelling marks the sixth time a U.N. school has been targeted by the Israeli military during its latest offensive in the Gaza Strip, and the second time such an institution has been destroyed.
"We have visited the site and gathered evidence," Krahenbuhl's statement said, adding that "our initial assessment is that it was Israeli artillery that hit our school, in which 3,300 people had sought refuge. We believe there were at least three impacts."
The Israeli military said it was still investigating the incident, and in a brief statement said Palestinian fighters near the facility had fired mortar bombs and Israeli forces had shot back.
"Earlier this morning, militants fired mortar shells at [Israeli] soldiers from the vicinity of the UNRWA school in Jabaliya. In response, soldiers fired towards the origins of fire, and we're still reviewing the incident," a military spokeswoman said.