Israel launched fresh air strikes against central Gaza on Tuesday, where the United Nations said it was alarmed by an intensification of Israeli attacks that had killed more than 100 Palestinians in one part of the enclave since Christmas Eve. The days around Christmas have seen an upsurge in the war, particularly in a central area just south of the seasonal waterway that bisects the Gaza Strip. Israeli troops told civilians to leave the area, though many say there is no safe place left to go.