French telecoms operator Iliad has made a $15 billion cash offer to purchase 56.6 percent of T-Mobile USA, the No. 4 U.S. mobile provider, an unexpected move that counters an earlier merger proposal by Sprint. T-Mobile confirmed to CNBC that it received a proposal from Iliad, but said it had no further comment on the matter at this time. Iliad submitted the bid to T-Mobile's board of directors and said it is working with leading international banks to secure financing for the deal. "The U.S. mobile market is large and attractive," Iliad said in a statement. "T-Mobile US has successfully established a disruptive position, which in many ways, is similar to the one Iliad has built in France." Read MoreJust one phone? No, T-Mobile bets you'll get more



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The company said it values the remaining 43.4 percent of company at $40.5 per share on the basis of $10 billion of synergies to the benefit of the T-Mobile U.S. shareholders. Dow Jones first reported the news, citing a an unnamed source who said Iliad views the bid as a "one-time opportunity to enter the world's-largest telecoms market." Iliad has disrupted the French mobile and broadband market over the past few years with its ultra-cheap subscriber plans. Watch: Is telecom undervalued?

Japan's SoftBank, which controls Sprint, and T-Mobile owner Deutsche Telekom AG had reportedly agreed to broad terms of a merger between the two firms, under which Sprint would pay around $40 per share for T-Mobile, valuing the smaller rival at nearly $32 billion, according to Reuters, who said the deal was expected to occur sometime after September. S&P Capital IQ's Angelo Zino has a hold recommendation on T-Mobile and he believes Sprint can top the Iliad's offer, but says a merger of the two companies would endure intense scrutiny.