A statement by the spokesman for Ban Ki-moon expressed the UN chief's "outrage and regret" at the placement of weapons at a site run by the global organization. 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 e-mail yesterday evening.
The Islamic militant group Hamas controls Gaza. The US, Israel and the European Union all consider Hamas a terrorist organization, 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."