By Doina Chiacu WASHINGTON (Reuters) - Israeli Prime Minister Benjamin Netanyahu rebuffed international calls for a ceasefire while defending his country's offensive in Gaza during appearances on U.S. television on Sunday. Netanyahu appealed for sympathy for Israelis under siege from militant rockets as a warning siren followed by an all-clear signal punctuated his interview on CBS' "Face the Nation." "When we began this interview we were under bomb alert and as the minutes passed now we're told people can go out into the open air again," he said. Netanyahu's television appearances came as thousands of Palestinians fled their homes in a Gaza town after Israel warned them to leave before threatened attacks on rocket-launching sites. Militants in the Gaza Strip kept up rockets salvoes deep into the Jewish state and the worst bout of Israel-Palestinian bloodshed in two years showed no signs of abating as Western foreign ministers meeting in Vienna said a ceasefire was an urgent priority.