The United Nations is very disturbed about an incident last month in which Gaza terrorists fired mortar shells from a UN school and will open an investigation, a UN official in Jerusalem said Friday.

The official made the comment a day after UN Secretary-General Ban Ki-Moon condemned the militants for using the school.

Footage taken by an Israeli aircraft on October 29 and released by the IDF showed Palestinian militants firing mortars from the yard of an elementary school for boys in the north Gaza town of Beit Hanoun. The school is run by the UN Relief and Works Agency (UNRWA), which oversees aid to Palestinian refugees.

"The United Nations is very disturbed about this, and wants to make sure that such incidents are not repeated. It is unacceptable that UN premises be used in this fashion," the UN official said.

The official spoke on condition of anonymity because he was not authorized to release full details of the incident to the press.

The school's students and guards were evacuated earlier in the day because of an IDF incursion nearby, the official said, allowing the terrorist to gain access to the grounds.

Ban has ordered UNRWA to investigate the incident, he said.

The black-and-white Israeli footage, taken from the air using thermal imaging equipment, shows three figures firing mortars next to the three-story school building, running to take cover after each shell. They are then seen taking the mortar launcher and leaving the school grounds.

The army said it "targeted and hit" the squad after it left the premises, but this is not depicted in the video.

It was not clear which of Gaza's factions was responsible for launching the mortars from the school.

On Thursday, Ban condemned the terrorists' actions, which he called "a serious violation of the UN's privileges and immunities," according to a statement on the UN web site.

Ban called on "all involved in this conflict to avoid actions that endanger the lives of civilians, especially children, and that put at risk UNRWA's ability to carry out its humanitarian mission," the statement read.