Japanese wireless service provider Softbank Corp is looking to buy roughly 70 percent of Sprint Nextel Corp in a bold move that would make it a major player in the U.S. mobile market.

But Softbank's ambitions may not stop with Sprint, which might also be looking to buy out its partner, Clearwire Corp. The Japanese company might also be aiming to use Sprint as a vehicle to make a run at smaller Sprint peer MetroPCS Communications Inc, a two-step transaction that would potentially cost more than 2 trillion yen ($25.55 billion), according to a Nikkei report.

That would make it the biggest overseas acquisition by a Japanese company ever and vault Softbank into the upper echelons of wireless carriers worldwide.

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 U.S. 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) and is in talks with several banks to borrow money to finance a bid, according to a source with direct knowledge of the matter.