NEW : Death toll in U.S. since airstrikes began is at At least four . NEW : U.N. secretary-general says ground assault the Middle East peace efforts . Hamas aim is to halt Hamas rockets , not regime change , spokesman says . Incursion 's spokesman : `` Gaza will be your cemetery ''
NEW : Death toll in Gaza since airstrikes began is at 460 . NEW : U.N. secretary-general says ground assault mars peace efforts . Incursion 's aim is to halt Hamas rockets , not regime change , spokesman says . Hamas spokesman : `` Gaza will be your cemetery ''