Warren Buffett's Berkshire Hathaway Inc on Saturday signaled he plans to make more large acquisitions to expand a conglomerate that posted a record profit in 2013, helped by a recovering U.S. economy.

"Charlie and I have always considered a 'bet' on ever-rising U.S. prosperity to be a very close to a sure thing," Buffett, 83, said in his annual letter, referring to his 90-year-old vice chairman Charlie Munger. "America's best days lie ahead."

Full-year profit rose 31 percent to $19.48 billion, or $11,850 per Class A share, while operating profit rose 20 percent to $15.14 billion, or $9,211 per share.

For the fourth quarter, net profit rose nearly 10 percent to $4.99 billion, or $3,035 per share, and operating profit rose 34 percent to $3.78 billion, or $2,297 per share.

Results improved in many of the Omaha, Nebraska-based company's more than 80 businesses, from retail to railroads, insurance to ice cream.

"On the operating front, just about everything turned out well for us last year - in certain cases very well," the world's fourth-richest man wrote to shareholders.

Buffett said he remains on the hunt for more big acquisitions, which he calls "elephants," after two recent big purchases: a $5.6 billion acquisition of Nevada utility NV Energy by Berkshire's MidAmerican Energy unit, and a $12.25 billion investment in ketchup maker H.J. Heinz Co.

Buffett has long recommended diversified investments in basic businesses capable of steadily boosting revenue over the long haul.

While Berkshire is known for its insurance businesses, Buffett said profit from a "Powerhouse Five" of large non-insurance unit - the BNSF railway, Iscar for metalworking, Lubrizol for chemicals, Marmon for industrial products, and MidAmerican - might boost pre-tax profit in 2014 by $1 billion from the $10.8 billion they collected in 2013.

Book value per share at the end of 2013 rose 18.2 percent from the end of 2012, to reach $134,973.

(Reporting by Luciana Lopez and Jonathan Stempel; Editing by Sophie Hares, Jennifer Ablan and Marguerita Choy)