REUTERS - Japanese mobile operator Softbank Corp said it will buy about 70 percent of Sprint Nextel Corp for $20.1 billion, giving Softbank the American toehold it has long desired and Sprint the capital to expand its network and potentially buy peers.

The deal for the third-largest U.S. wireless carrier represents the most a Japanese firm has spent on an overseas acquisition.

Announced by Softbank's billionaire founder and chief Masayoshi Son and Sprint Chief Executive Dan Hesse at a packed news conference in Tokyo on Monday, the transaction gives Softbank entry into a U.S. market that is still growing, while Japan's market is stagnating.