TORONTO (Reuters) - Gold miner Kinross said on Thursday it has agreed to acquire exploration company Underworld Resources in a deal worth C$139.2 million ($135.3 million), expanding its asset base in northwest North America.

Underworld is focused on exploration in Canada. It's flagship asset is the White Gold project, located in the Tintina gold belt in Canada's Yukon Territory. The project has indicated and inferred resources of about 1.5 million ounces.

Kinross owns the Fort Knox open pit mine in neighboring Alaska, which is expected to produce about 370,000 ounces of gold annually over the next five years.

The acquisition would seem to fit well with Kinross's stated goal of expanding its asset base in the American Cordillera, the chain of mountain ranges that form the western backbone of North America, Central America and South America.

Toronto-based Kinross is offering 0.141 of a Kinross common share, plus C$0.01 in cash for every share of Underworld. The offer represents a value of C$2.62 a share, a 36 percent premium to Underworld's closing price on March 10.

Kinross will issue about 6.8 million shares, or 1 percent of its current outstanding common shares, to pay for Underworld. Kinross already owns roughly 8.5 percent of Underworld's outstanding shares.

The deal, which has been unanimously approved by Underworld's board of directors, is subject to shareholder and regulatory approvals.

($1=$1.0286 Canadian)

(Reporting by Euan Rocha; editing by Peter Galloway)