U.S. regulators on Friday closed Texas lender Guaranty Bank and sold its assets to Banco Bilbao Vizcaya Argentaria , allowing Spain's second-largest bank to expand its reach in the U.S. market.

Guaranty, a unit of Guaranty Financial Group Inc , is the 81st U.S. bank failure this year and represents another sizable hit to the Federal Deposit Insurance Corp's fund used to protect customer accounts.

Guaranty had assets of about $13 billion and deposits of $12 billion, and will cost the FDIC's insurance fund about $3 billion, the agency said.

It is about half the size of Colonial Bank, which last week became the largest failure of 2009, with assets of $25 billion, the most dramatic among a recent surge in bank failures, especially among larger regional banks.

BBVA bought the assets of Guaranty through Alabama-based bank BBVA Compass, which BBVA bought in 2007 and resulted in the Spanish bank more than tripling its U.S. branch base.

Guaranty was the largest of four bank failures on Friday. Regulators also closed three small banks: Atlanta, Georgia-based ebank; Newman, Georgia-based First Coweta; and Birmingham, Alabama-based CapitalSouth Bank.

In total, the four banks are expected to cost the FDIC's insurance fund $3.3 billion.

The agency entered into a loss-share agreement with BBVA Compass in which the FDIC agreed to share the risk that a pool of $11 billion of Guaranty's assets could further deteriorate.

Guaranty is considered a desirable buy for BBVA as it tries to extend its reach into the Spanish-speaking market of the United States, which complements its operations in Mexico where BBVA owns the country's biggest bank, Bancomer.

At the time of closing, Guaranty operated 162 branches in Texas and California with a focus on real estate construction lending and financing to middle-market businesses.

(Reporting by Karey Wutkowski; Editing by Marguerita Choy)