U.S. wireless carriers T-Mobile US Inc (TMUS.O) and Sprint Corp (S.N) are said to be in active merger talks, CNBC reported on Tuesday, citing sources.

Japan's SoftBank Group Corp, which owns a majority stake in Sprint, and T-Mobile parent Deutsche Telekom have been in frequent conversations about a stock-for-stock merger, CNBC reported.

Sprint's shares rose 6.8 per cent, while T-Mobile's shares were up nearly 4.6 per cent in morning trade.

CNBC reported that the companies are still weeks away from finalizing a deal and believe the chances of reaching that deal are not assured. 

With the all-stock nature contemplated, SoftBank would emerge as a large minority holder in any combination, CNBC reported.

While T-Mobile Chief Executive John Legere is expected to lead any combination that results from a merger, SoftBank Chief Executive Masayoshi Son has made it clear he wants a say in how the company is run, CNBC said.

Reuters reported in February that SoftBank was prepared to give up control of Sprint to T-Mobile to clinch a merger of the two U.S. wireless carriers.

Over three years ago, SoftBank abandoned talks to acquire T-Mobile for Sprint amid opposition from U.S. antitrust regulators. That deal would have put SoftBank in control of the merged company, with Deutsche Telekom becoming a minority shareholder.

Since then, T-Mobile has overtaken Sprint in market capitalization - the company is valued at about US$51 billion, while Sprint has a market value of about US$30 billion.

Sprint was also said to be in talks with cable providers Charter Communications Inc (CHTR.O) and Comcast Corp (CMCSA.O) for a wireless partnership, according to reports.

Last month, Sprint's chief executive said an announcement on merger talks should come in the "near future".

Both Sprint and T-Mobile did not immediately respond to Reuters' requests for comment, while Deutsche Telekom declined to comment.