(Corrects to Aden, instead of Yemen, in para 6) * Saudi-led jets target Houthi-allied Republican Guards * Shuttle diplomacy continues in Geneva, extension eyed * UN makes revised $1.6 billion appeal for aid to civilians SANAA/GENEVA, June 19 (Reuters) - Saudi-led warplanes bombed elite Republican Guard forces allied with the dominant Houthi faction in Yemen's civil war on Friday, residents said, as U.N. diplomats in Geneva struggled to nudge the various sides towards a ceasefire deal. More than 2,600 people have been killed since an Arab alliance led by Saudi Arabia launched air strikes to try to stop the Iranian-backed Houthis from completing a takeover of Yemen and to try to reinstate exiled President Abd-Rabbu Mansour Hadi.