SYDNEY, Jan 11 (Reuters) - Singapore Airlines (SIAL.SI)is aiming to raise $500 million to $750 million in a U.S. dollar bond deal, according to two sources with direct knowledge of the matter. The sources could not be named as the information was not yet made public. Singapore Airlines did not immediately respond to a request for comment. The airline has mandated banks to work on the transaction and will start briefings with investors about the deal on Tuesday, a term sheet seen by Reuters showed. Citigroup Inc (C.N) and DBS (DBSM.SI) are leading the transaction, with BNP Paribas (BNPP.PA) and Standard Chartered (STAN.L) as book runners, the term sheet showed. The deal is classified as Reg S, which means the bonds can only be purchased by investors outside of the United States.