UN secretary-general has said he was "alarmed" to hear that rockets placed in a Gaza school "have gone missing" and demanded a full review of the incident.

Ban Ki-moon's spokesman expressed "outrage and regret" at the storing of weapons at a UN-run site. The UN refugee agency for Palestinians (UNRWA) said a similar incident has happened twice in a week.

"UNRWA strongly and unequivocally condemns the group or groups responsible for this flagrant violation of the inviolability of its premises under international law," the agency said. In the latest incident, the weapons were found between two other facilities that currently houses around 3,000 displaced Palestinians. It said in a statement that because staff were withdrawn quickly, they were "unable to confirm the precise number".

UNRWA was reported to have found 20 rockets and handed them over to the local authorities "in accordance with standard practice". The move caused a backlash in Israel because that could mean Hamas, which controls and administers the Gaza Strip, may now have access to them.

Critics in Israel said the missiles "were passed on to the government authorities in Gaza, which is Hamas". But UNRWA spokesman Chris Gunness denied the rockets were in the possession of the Islamist group.

However, Ban's deputy spokesman Farhan Haw said in an email that since the rockets were handed back to local authorities "they have gone missing".

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