U.S. wireless carriers T-Mobile US Inc and Sprint Corp are said to be in active merger talks, CNBC reported, 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 were up 6.7 percent, while T-Mobile shares rose nearly 3 percent 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.

Both Sprint and T-Mobile did not immediately respond to Reuters' requests for comment.