Israeli shelling of the United Nations headquarters in Gaza drew strong condemnation Thursday from U.N. Secretary General Ban Ki-moon who is in Israel pushing for an immediate end to hostilities. “Today, the U.N. compound in Gaza has been shelled again. I conveyed my strong protest and outrage to the Defense Minister [Ehud Barak] and to the Foreign Minister [Tzipi Livni], and demanded a full explanation,” Ban said at press conference in Tel Aviv, according to a transcript provided by his spokesperson's office in New York. Ban said Barak told him “it was a grave mistake and that he took it very seriously.” “He assured me that extra attention would be paid to the U.N. facilities and staff and that this should not be repeated,” Ban said. The shelling of the United Nations Relief and Works Agency headquarters in Gaza is the fifth attack on U.N. facilities in the 20-day Israeli offensive. The shelling resulted in uncontrollable fires to the building that was being used as a temporary shelter for Palestinian civilians. Three UNRWA staff were reported injured in the attack on the building, which was also used to store aid supplies.