The U.N. secretary-general on Wednesday said he was "alarmed" to hear that rockets were placed in a U.N.-run school in Gaza and now "have gone missing," and he demanded a full review of such incidents.

A statement by the spokesman for Ban Ki-moon expressed the U.N. chief's "outrage and regret" at the placement of weapons at a site run by the global organization. The U.N. says that has happened at least twice so far in the current fighting.

"Those responsible are turning schools into potential military targets, and endangering the lives of innocent children," U.N. staff and anyone seeking shelter, the statement said.

The rockets had been placed at one of the schools run by the U.N. refugee agency for Palestinians, UNRWA.

Once they were found, "in accordance with standard practice, UNRWA handed them over to the local authorities. Since then, they have gone missing," Ban's deputy spokesman, Farhan Haq, said in an email Wednesday evening.

The Islamic militant group Hamas controls Gaza. The U.S., Israel and the European Union all consider Hamas a terrorist organization, but the U.N. does not.

A week ago, UNRWA said that during a routine check it discovered about 20 rockets hidden in one of its vacant Gaza schools and called on militants to respect the "sanctity and integrity" of U.N. property. It said the incident was "the first of its kind in Gaza."

On Tuesday, UNRWA reported a second incident, saying it found rockets hidden at a vacant school during a regular inspection. "UNRWA staff were withdrawn from the premises, and so we are unable to confirm the precise number of rockets," its statement said. "The school is situated between two other UNRWA schools that currently each accommodate 1,500 internally displaced persons."

The statement said UNRWA was looking at all possible ways to safely remove the rockets and would investigate the incident.

The U.N. statement Wednesday said Ban has asked for the immediate development of a plan to safely handle any weapons found on U.N. premises, and he told the U.N. Mine Action Service to immediately send people to deal with the situation of the missing rockets.

"The United Nations is taking concerted action to increase its vigilance in preventing such episodes from happening again," the statement said.

The statement also demanded that militant groups stop such actions and be held accountable for endangering civilians.

UNRWA tweeted Monday that more than 83,000 people were now taking refuge in its schools in Gaza, and "numbers are growing."