Japanese mobile carrier Softbank Corp may buy a majority stake in Sprint Nextel Corp to establish a foothold in the U.S. wireless market, in what could be the largest Japanese acquisition ever in America.

In response to reports of a pending deal, Sprint said on Thursday that it was in talks with Softbank on a "potential substantial investment" that could involve a change in control of the company. It said there was no assurance of a sale.

Softbank is eyeing a controlling stake in Sprint worth more than 1 trillion yen ($12.8 billion), according to a source with direct knowledge of the matter, adding that the Japanese company is in talks with several banks to borrow money to finance a bid.

A second source familiar with the situation, who declined to speak publicly about the matter, said Softbank has been exploring ways to get into the U.S. market since this summer, as it sees opportunities for growth here that would help offset a stagnating market in Japan.

Sprint shares rose as much as 19 percent to levels not seen since the summer of 2011, on the heaviest volume in the stock's history. The shares of wireless carrier Clearwire Corp, which could play a key role in any deal, rose 38 percent.

"The addition of Softbank would provide a deep-pocketed partner - removing any financing risk and meaningfully reducing (Sprint's) ongoing cost of capital," Evercore analyst Jonathan Schildkraut said.

At $12.8 billion, the deal would be almost one-third larger than the previous top Japanese deal in the United States, NTT Docomo's 2000 investment in AT&T Wireless.

DEALS MULTIPLYING

Sprint, whose market capitalization was $15.12 billion at Wednesday's market close, is the third-largest U.S. carrier, with more than 56 million users at the end of June. It is in the middle of a costly network upgrade that has led it to consider a range of partnerships.

One analyst said Sprint might be the Japanese company's only option if it is eyeing the American market.

"In terms of (Sprint) standalone, we believe the asset represents the only way for a potential new entrant to get a national presence immediately in the U.S.," Wells Fargo analyst Jennifer Fritzsche wrote in a note to clients.

The Softbank news comes just days after a source told Reuters that Sprint has been considering whether to make a bid for smaller rival MetroPCS Communications Inc, which agreed this month to merge with Deutsche Telekom AG's and NTT Docomo.

As it chases market share, Softbank said this month it would buy smaller mobile service operator eAccess Ltd in a $1.84 billion deal. It said the buy would give it a total of 39 million users, just ahead of KDDI's 36 million.

Japanese media said buying Sprint - which competes in the United States against Verizon Wireless and AT&T Inc - would also make it cheaper for Softbank to procure smartphones and other mobile devices.

Wells Fargo's Fritzsche said regulators would likely look favorably upon a deal that would bring an outside international player to the United States.

Japanese companies made a record 642 cross-border deals last year, according to Thomson Reuters data. Buoyed by a stronger yen, the value of all overseas deals rose to $69.5 billion, up 81 percent from 2010, also a record.

(Additional reporting by Mari Saito and James Topham in Japan, Sruthi Ramakrishnan in Bangalore and Carey Gillam in Overland Park, Kansas; Writing by Ian Geoghegan and Ben Berkowitz; Editing by Ron Popeski, Bernadette Baum, John Wallace and Andre Grenon)