The UN secretary-general says he is "alarmed" to hear rockets were placed in a UN-run school in Gaza and now "have gone missing".
A spokesman for Ban Ki-moon says he has demanded a full review of such incidents.
In a statement released on Wednesday night Ban expressed "outrage and regret" at the placement of weapons at a site run by the global organisation.
The UN 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," UN staff and anyone seeking shelter, the statement said.
The rockets had been placed at one of the schools run by the UN refugee agency for Palestinians, the United Nations Relief and Works Agency in Gaza.
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.
The Islamic militant group Hamas controls Gaza. The US, Israel and the European Union all consider Hamas a terrorist organisation, but the UN 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 UN 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"UNRWA staff were withdrawn from the premises, and so we are unable to confirm the precise number of rockets," it said.
"The school is situated between two other UNRWA schools that currently each accommodate 1500 internally displaced persons."
UNRWA said it was looking at all possible ways to safely remove the rockets and would investigate the incident.
The UN statement on Wednesday said Ban has asked for the immediate development of a plan to safely handle any weapons found on UN premises, and he told the UN Mine Action Service to immediately send people to deal with the situation of the missing rockets.
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