Softbank's ambitions in the US may not stop with just an acquisition of Sprint, following reports that the Japanese company might use Sprint as a vehicle to buy smaller US mobile provider MetroPCS, which this month agreed to merge with Deutsche Telekom’s T-Mobile USA.

The Nikkei newspaper reported today that the two-step deal would potentially cost more than JPY2 trillion (US$25.6 billion), making it the biggest overseas acquisition by a Japanese firm and propelling Softbank into the ranks of the top mobile operators worldwide.

Reuters reports that Softbank is in talks with three major Japanese banks to borrow around JPY1.8 trillion (US$23 billion) to finance its initial plans to acquire Sprint. However, at least one brokerage is concerned that a deal of this size could leave Softbank with unacceptable levels of debt.

Softbank has net debt of about US$10 billion, while Sprint has debt of US$15 billion. A client note from Societe Generale said that adding the US$2 billion net debt of smaller rival eAccess (which Softbank last week agreed to to buy), would raise the new company’s “post-deal gearing levels to unacceptable heights.”

Meanwhile some analysts are questioning the business logic of combining Japan's number two wireless operator with the number three player in the United States.

Shares in Softbank fell 16.9 percent in Tokyo after confirmation of takeover talks with Sprint, while Sprint shares closed 14.3 percent higher in the US.