By Jonathan Stempel

(Reuters) - Warren Buffett's Berkshire Hathaway Inc <BRKa.N> is seeking permission from the Federal Reserve to increase its ownership stake in Wells Fargo & Co <WFC.N>, after reaching the 10 percent level that could prompt increased regulatory scrutiny.

According to papers obtained by Reuters on Friday, Berkshire said it learned in mid-March that its Wells Fargo stake, including 2.01 million shares held by Buffett, had reached 10.01 percent because of buybacks by the San Francisco-based bank, which decreased the number of shares outstanding.

Though the stake later dipped to 9.97 percent, Berkshire said it has again risen above 10 percent because of more buybacks by Wells, which is the third-largest U.S. bank by assets. Berkshire said it has bought no Wells Fargo shares since Oct. 21.

"Berkshire is seeking permission to retain its current ownership position in Wells Fargo and to acquire additional shares of common stock of Wells Fargo for investment purposes," according to the papers, which were dated June 14 and submitted to the Federal Reserve Bank of San Francisco.

The Federal Reserve exerts special oversight when investors take large bank stakes.

Berkshire said it had no dollar value in mind for purchases, had no plans to make significant changes to Wells Fargo's strategy or corporate structure, and was not contemplating changes to the bank's management or board.

Bloomberg News earlier reported the paperwork.

Berkshire's investment in Wells Fargo is worth more than $23 billion.

"We value Berkshire Hathaway as a long-term shareholder and customer, and we appreciate the confidence that Berkshire’s executive team has shown," Wells Fargo spokesman Ancel Martinez said an email.

Buffett has long been a supporter of Wells Fargo, calling it "very well run" and a "terrific operation," and saying that Chief Executive John Stumpf has done a "fabulous job."

The Federal Reserve exerts special oversight when investors take large bank stakes.

In a September 2008 policy statement, the Fed said it often lets investors take double-digit stakes not designed to exert a "controlling influence," but would review any resulting business relationships "case-by-case."

The U.S. Treasury Department has said it will not deem a 10 percent stake to result in "control" of a big bank if an investor agrees in writing to limits on its involvement.

Berkshire also owns 15.94 percent of American Express Co <AXP.N>, the application and Reuters data show.

Based in Omaha, Nebraska, Berkshire also operates close to 90 businesses such as auto insurer Geico, the BNSF railroad, Dairy Queen ice cream and various apparel, energy and industrial companies.

(Reporting by Jonathan Stempel in New York; Editing by Marguerita Choy and Leslie Adler)