The White House condemned the shelling Wednesday of a U.N. school in the Gaza Strip that was sheltering Palestinians displaced by the fighting between Israel and Hamas militants in Gaza.

The U.S. didn't assign responsibility for the shelling, but Gaza officials say Israel struck the school, killing 15 and wounding 90. Israeli's military said it fired back after soldiers were targeted by mortar rounds launched from the vicinity of the school.

White House spokeswoman Bernadette Meehan also said the U.S. is "extremely concerned" that thousands of Palestinians aren't safe in U.N.-designated shelters, despite being told by Israel's military to leave their homes. Israel has been warned civilians by phone and leaflet to leave dangerous areas ahead of strikes on militant targets.

At the same time, Meehan said the U.S. condemns those who have been hiding weapons in U.N. facilities. She reiterated the U.S. call for a quick cease-fire to stem the violence.

"All of these actions, and similar ones earlier in the conflict, are inconsistent with the U.N.'s neutrality," Meehan said. "This violence underscores the need to achieve a cease-fire as soon as possible."

Israel accuses Hamas militants of concealing weapons in U.N. facilities and of launching rockets from crowded residential areas, endangering civilians when Israel strikes back.

The sharp criticism from the U.S. comes amid an escalation of the violence in Gaza, where an Israeli airstrike hit a crowded Gaza shopping area on Wednesday, killing at least 16 people and wounding more than 150. Israel and Gaza's Hamas rulers have been stepping up attacks after more than three weeks of fighting that has killed more than 1,300 Palestinians and more than 50 Israelis.

Earlier at the United Nations, top officials expressed shock and outrage at an attack on a U.N. school, with Deputy Secretary-General Jan Eliasson repeating the U.N.'s call for an immediate cease-fire.