By Jonathan Stempel

NEW YORK (Reuters) - Fifth Third Bancorp (FITB.O: Quote, Profile, Research), a large U.S. Midwest regional bank, has sued First Horizon National Corp (FHN.N: Quote, Profile, Research), accusing Tennessee's biggest bank of backing out of an agreement to sell it nine branches in the Atlanta area.

The lawsuit was filed on Friday in the U.S. district court in Cincinnati, where Fifth Third is based.

In its complaint, Fifth Third accused First Horizon of "willfully and wrongfully" refusing to close, after demanding that it pay twice as much as the maximum amount contemplated when the banks agreed on the transaction last Sept. 21.

Terms were not available, but Fifth Third characterized the banks' agreement as a "substantial multimillion dollar contractual relationship."

Fifth Third said it has incurred more than $5 million of related costs, including for employee training, mailings, and replacement of computers and signage. It asked the court to enforce the sales agreement.

First Horizon spokesman Anthony Hicks said his company does not discuss pending litigation.

Charles Burkett, president of Tennessee and national banking at Memphis-based First Horizon, earlier Friday said the dispute surfaced late in the merger process.

"This really came to the forefront on (Thursday) the 7th," Burkett said in an interview.