The talks come just a week after Sprint rival T-Mobile USA Inc. agreed to combine with MetroPCS Communications Inc.

Japanese broadcaster NHK reported that the talks centered upon Softbank taking a majority stake in Sprint for upward of $12.8 billion. Softbank, a Tokyo-based Internet and technology company, would acquire about 75% of Sprint as well as at least Sprint's share in Clearwire Corp., a U.S.-based wireless Internet service provider, NHK reported. It is unclear whether the $12.8 billion figure would be just for Sprint, or for some or all of Clearwire as well.

Sprint had reportedly held talks with both MetroPCS and T-Mobile parent Deutsche Telekom AG in recent months, as second-tier U.S. wireless companies look to bulk up, the better to take on giants Verizon Communications Inc. and AT&T Inc.

Already trading at a robust valuation, Sprint gained 66 cents, or 13%, to stand at $5.70 on Thursday morning, following reports of Softbank's interest.

Chris King of Stifel Nicolaus & Co. observed that "we do not view the stock as compellingly inexpensive," noting in a Thursday report that even before Thursday's surge, Sprint had been trading at nearly 6 times projected 2013 Ebitda. It was closer to 7 times when factoring in the company's holdings in wireless broadband wholesaler Clearwire, which gained 39 cents, or about 30%, to $1.69 on Thursday morning.

A combination of Softbank and Sprint would not provide the savings and other benefits of merging two U.S. carriers.

However, Softbank uses the same version of long-term evolution, or LTE, technology as Clearwire, in which Sprint holds a majority position. The standard is known as time division LTE, or TD-LTE. Clearwire also has a massive stash of 2.5 GHz spectrum, which Softbank has used to deploy TD-LTE.

Moreover, Jonathan Schildkraut, an analyst at Evercore Partners Inc., noted Thursday that Softbank and Clearwire are partners in the Global TD-LTE Initiative to develop the technology.

Over the years, Sprint has been punished for its majority stake in Clearwire, and the uncertainty surrounding the upstart wireless broadband provider's funding. For the moment, at least, Sprint is enjoying a Clearwire premium.

According to the reports, talks between Sprint and Softbank originally also involved T-Mobile and Deutsche Telekom, but the Germans eventually backed out due to fears over U.S. antitrust concerns and instead negotiated a merger with MetroPCS.

Softbank persisted, reportedly hopeful to amass a large portfolio of wireless spectrum in the U.S. to be used to offer data as well as voice services.

Softbank, led by chief executive Masayoshi Son, has shown an appetite for telecom deals, agreeing Oct. 2 to acquire eAccess Ltd. of Japan for $2.3 billion to boost its network capacity in its home market. Softbank entered the wireless market in Japan in 2006 via the purchase of Vodafone Group plc's operations in that country.

Sprint has been pondering its next move in the days since the T-Mobile-MetroPCS deal was announced. The company last week said that outgoing head of M&A Keith Cowan would delay his exit from the company amid reports that Sprint could be plotting a counterbid for MetroPCS.

Shares of MetroPCS dropped more than 7% in the premarket on trader speculation that a Sprint-Softbank deal would eliminate any chance that Sprint makes an offer for that company.

Sprint is attempting a costly upgrade of its wireless network, including finally fully integrating its $35 billion purchase of Nextel Communications Inc. Sprint bought Nextel in 2005, but has kept the target's network up and running while it worked on its next-generation offering.