NEW : Prime Minister Fouad Siniora calls attack a violation of U.N. resolution . NEW : Lebanese government , United Nations investigating , Fouad Siniora says . at least four Israel -based rockets strike northern Lebanon , authorities say . Lebanese military returns fire toward source of rocket attack .
NEW : Prime Minister Fouad Siniora calls attack a violation of U.N. resolution . NEW : Lebanese government , United Nations investigating , Siniora says . Four Lebanon-based rockets strike northern Israel , authorities say . Israeli military returns fire toward source of rocket attack .