Gaza Strip ? Israeli artillery shells struck the U.N. headquarters in the Gaza Strip on Thursday, setting a food warehouse ablaze and drawing a sharp rebuke from the visiting U.N. chief who called it an “outrage.” Another Israeli bombardment killed Hamas’ head of security.

The attack added to a day of deadly chaos pitting Israeli troops against Islamic militants. Terrified residents huddled in shelters and stairwells, or scooped up toddlers and fled on foot.

After nightfall, shells landed near Gaza City’s Quds Hospital, where many families had sought refuge, and the building caught fire, forcing staff to evacuate hundreds of people. According to a hospital medic, some patients were pushed down the street on gurneys; a few held white flags.

The destruction added to what aid groups say is a humanitarian crisis in Gaza and ratcheted up tensions between Israel and the international community even as diplomats indicated progress in cease-fire talks.