UNITED NATIONS — A senior United Nations official on Thursday forcefully rebuffed suggestions that the organization’s lax security had allowed militants to use its facilities in Gaza as a weapons cache and expressed outrage that three of its schools had been hit by shelling in as many days this week, even though the Israeli military knew their precise locations.

John Ging, the director of operations for the United Nations Office for the Coordination of Humanitarian Affairs, said in an interview here that his organization had protested vigorously to both sides.

He added that the United Nations had given the Israeli military precise GPS locations of all 83 schools that were being used as shelters for 141,000 people who had fled their homes. On Tuesday, one school was hit, wounding one person. On Wednesday, another was hit, wounding five. On Thursday, 16 were killed, the majority women and children, at the school in Beit Hanoun.

“There is no justification for the loss of so many innocent civilian lives,” Mr. Ging said.

He wondered aloud how Israel’s “sophisticated weaponry” could allow known United Nations locations to come under shelling, though he said the agency did not yet fully know who fired the shells and from where.