Israel unleashed its largest shelling of Gaza's crowded neighborhoods on Thursday, hitting a U.N. compound, a hospital and a media building leaving 15 Palestinians killed and still counting.

U.N. Secretary-General Ban ki-Moon, who was in Israel for the day, called the attack an outrage and demanded a full explanation.

[Israeli] Defense Minister Barak said to me it was a grave mistake and he took it very seriously, Ban said at a news conference in Tel Aviv with Israeli Foreign Minister Tzipi Livni, according to CNN.

Israeli forces moved into Gaza City overnight in search of Hamas fighters. Israeli troops are being backed by helicopter gunships, tanks and heavy guns.

The Palestinian death toll from the air-and-ground offensive has risen to at least 1,073 and there were more than 5,000 wounded, according to the Hamas-run Health Ministry in Gaza.

A Palestinian rights group said at least 670 of the dead were civilians.

Meanwhile, thirteen Israelis have been killed, 10 soldiers and three civilians hit by Hamas rocket fire since Israel launched on December 27 a military campaign.