Witnesses say an Israeli airstrike has collapsed a 12-story apartment tower in central Gaza City.Gaza police say Israeli aircraft fired a warning missile at the roof of the tower at dusk Saturday, followed five minutes later by two missiles with explosives.The blast sent a huge ball of fire and a black cloud of smoke into the air. The explosions shook neighboring buildings.Gaza health official Ashraf al-Kidra says at least 10 people, including four children, were wounded.