Palestinian militants fired hundreds of rockets from the Gaza Strip into Israel on Wednesday, while Israel pressed ahead with a series of airstrikes that have killed 21 Palestinians, including three senior militants and at least 10 civilians. The rocket fire set off air-raid sirens throughout southern Israel, where residents had been bracing for an attack since Israel carried out its first airstrikes early Tuesday. Earlier Wednesday, Israeli aircraft struck targets in Gaza a second straight day, pushing the region closer toward a new round of heavy fighting. Palestinian militants have pledged to retaliate while Israel says it is prepared for a further escalation of hostilities.