GAZA CITY, Gaza Strip (AP) - Israeli artillery shells crashed into the United Nations headquarters in the Gaza Strip on Thursday, drawing a sharp protest from the U.N. chief as he visited Israel to appeal for a truce. Another Israeli bombardment killed the Hamas security chief.

The deadly chaos pitting Israeli troops against Islamic militants transformed parts of Gaza City into a full-bore war zone. Terrified residents huddled in shelters and stairwells, or scooped up toddlers and fled on foot. Blasts and bullets tore chunks out of buildings.

The Israeli attack on the U.N. compound triggered a fire that engulfed a warehouse and destroyed thousands of pounds of food and other aid intended for Gaza's beleaguered citizens. Workers with fire extinguishers and Palestinian firefighters, some wearing bulletproof jackets, struggled to douse surging flames and tugged bags of flour from the debris.

The destruction delivered a sharp blow to efforts to ease what aid groups say is a full-fledged humanitarian crisis in Gaza, and ratcheted up tensions between Israel and the international community even as diplomats indicated progress in cease-fire talks.

Israeli envoy Amos Gilad returned from a daylong trip to Cairo, where he discussed a cease-fire proposal with Egyptian officials who are also trying to coax Hamas into ending the war.

Israel launched an offensive Dec. 27 to end Hamas rocket attacks on Israel. Gaza medics say about 1,100 Palestinians, half of them civilians, have died; 13 Israelis have also been killed.

Israel's intense assaults in Gaza on Thursday seemed to reflect an extra push to pressure Hamas negotiators into making concessions on cease-fire terms, and punish the militant group as much as possible before any end to hostilities.