Sprint says it is in talks with Japanese cellphone company Softbank about a potential substantial investment in the U.S. company.

Sprint Nextel Corp. said Thursday that it can’t assure the talks will lead to any transaction.

The No. 3 U.S. wireless carrier said it does not plan to give any more details unless it reaches an agreement with Softbank.

Japan’s third- largest mobile-phone company wants to buy control of Sprint, two people familiar with the matter told Bloomberg news before Sprint confirmed the talks.

Softbank is seeking two-thirds of Overland, Kansas-based Sprint, said one of the people, who asked not to be identified because the discussions are private. Softbank is in talks to acquire the stake for more than 1.5 trillion yen ($19 billion), Japan’s Nikkei newspaper reported on its website, without saying where it got the information.

The Japanese company could shift the balance of power in the U.S. telecommunications industry at a tumultuous time. Softbank would provide much-needed financial support for Sprint as the U.S. third-largest carrier tries to compete with bigger rivals Verizon Wireless and AT&T Inc. Sprint would give Softbank a base for entering the U.S. market with a compatible carrier via its joint venture with Clearwire Corp., said Jennifer Fritzsche, an analyst at Wells Fargo Securities LLC.

Sprint “represents the only way for a potential new entrant to get a national presence immediately in the U.S.,” Fritzsche wrote in a report. Softbank and Clearwire use airwaves in the same frequency and the same TDD LTE variant of network technology.

Clearwire Report

Softbank is also expected to buy Clearwire, CNBC reported. Clearwire’s shares soared 31 percent in early trading. Sprint jumped 20 percent to $6.04 at 8:14 a.m. before the market opened. Before today, the stock had more than doubled this year, giving Sprint a market value of $15.1 billion.

Softbank said it wasn’t the source of the report in a statement to the Tokyo Stock Exchange. Bill White, a spokesman for Sprint, declined to comment. Mike DiGioia, a spokesman for Clearwire, also declined to comment.

Sprint is holding off on an immediate counterbid for MetroPCS Communications Inc. to gain time to scrutinize Deutsche Telekom’s planned combination with T-Mobile USA, people familiar with the situation said yesterday. Deutsche Telekom’s shares climbed 1.2 percent in 15 minutes after the Nikkei report, a sign that investors may see a Softbank-Sprint deal increasing the chances for the T-Mobile merger with MetroPCS.

Softbank was the first carrier in Japan to offer Apple Inc.’s iPhone, which helped it boost earnings by more than sevenfold over the past four years. The Tokyo-based company, the fastest-growing Japanese mobile-phone provider, this month agreed to pay 180 billion yen for smaller local rival, eAccess Ltd.

(Bloomberg and The Associated Press contributed to this report.)

“It’s surprising that the company is considering M&A in the North American market,” said Shinji Moriyuki, an analyst at SMBC Nikko Securities in Tokyo. “If Softbank can use its marketing knowhow and data communications, with which the company has been successful in Japan, there will probably be synergies in North America.”

Founded by billionaire Masayoshi Son, Softbank entered the mobile phone business in 2006 by paying 2 trillion yen for Vodafone Group Plc’s Japanese unit.