Softbank agreed to buy a stake of about 70 percent in Sprint Nextel for $20.1 billion as Japan’s third-biggest mobile-phone operator seeks growth overseas amid a declining local market.

Softbank will pay $12.1 billion to Sprint shareholders, and the deal includes $8 billion of new capital.

The deal sparked speculation that the extra cash would allow Sprint to gain full control of wireless broadband firm Clearwire, in which it holds a 48 percent stake.

Sprint, based in Overland Park, Kansas, fell just under 1 percent to $5.69.

Clearwire soared 16 percent to $2.69.