PARIS (Reuters) - French low-cost telecommunications company Iliad SA confirmed on Thursday it had made a buyout offer for T-Mobile US Inc , rivaling an existing takeover offer from Sprint Corp . Confirming reports from earlier in the day it said in a statement it had offered $15 billion in cash for 56.6 percent of T-Mobile US at $33 per share. Iliad said it valued the rest of T-Mobile, the third-largest U.S. carrier and owned by Deutsche Telekom , at $40.50 per share and expects $10 billion of cost savings. Iliad, whose bid is backed by its billionaire founder Xavier Niel, said the proposed deal amounted to a premium of 42 percent premium over T-Mobile's share price before the announcement. The bid already has financing from unnamed international banks and would also require a capital increase, supported by Niel, it said. "There can be no certainty that the Iliad offer will be accepted by the board of directors of T-Mobile US," the company said. (Corrects paragraph three to say T-Mobile US is the fourth-largest U.S. carrier, not the third largest) (Reporting by Leila Abbound; Editing by Andrew Callus)