The talks, which were first reported by Japanese broadcaster NHK, come just a week after Sprint rival T-Mobile USA Inc. agreed to combine with MetroPCS Communications Inc.

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

Overland Park, Kan.-based Sprint had no immediate comment on the report, while Softbank officials said only the reports are "based on speculation." That speculation centers on Softbank, a Tokyo-based Internet and technology firm, acquiring upwards of 75% of Sprint as well as at least Sprint's share in Clearwire Corp., a U.S.-based wireless Internet service provider.

A deal announcement is believed to be weeks away, if it happens at all. It is unclear whether the $12.8 billion figure mentioned in Japanese reports would be just for Sprint, or for some or all of Clearwire as well.

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 has shown an appetite for telecom deals of late, 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.