Japanese mobile phone company SoftBank has agreed to buy 70 per cent of Sprint Nextel Corp for $US20 billion ($19.60 billion), according to a news report.

The deal is expected to be announced later today, according to CNBC's website, which cited anonymous sources "close to the situation".

The transaction could make Sprint, the third-largest US mobile phone company, a tougher competitor against its bigger rivals, Verizon Wireless and AT&T.

Sprint has 56 million subscribers, about half the number of Verizon Wireless, the leading US mobile phone company.

SoftBank will buy $US8 billion in shares from Sprint and another $US12 billion from current Sprint shareholders, the CNBC report said. SoftBank will offer $US7.30 a share, CNBC said, about 27 per cent above Friday's closing price of $US5.73.

John Taylor, a spokesman for Sprint, declined to comment beyond the company's statement last week confirming that it was in talks with SoftBank.

Word of the talks pushed up Sprint's stock more than 14 per cent on Thursday.

Sprint has struggled in recent years to compete with Verizon and AT&T. The company has $US21 billion in long-term debt, and has launched a costly network restructuring and signed a long-term contract to buy $US15.5 billion worth of iPhones from Apple over four years.

Tokyo-based SoftBank, once the underdog in Japan's telecom industry, has seen its fortunes improve since it started selling the iPhone in 2008. It was initially the only Japanese phone company to offer it.