OMAHA, Neb. –  Investor Warren Buffett says the U.S. economy continues to improve gradually, and he doesn't think any price "bubbles" are developing.

Buffett told Berkshire Hathaway shareholders at the company's annual meeting Saturday that American businesses are thriving as is evident in their profit reports.

The fact that interest rates have remained near zero for several years has some investors worried about bubbles forming in prices.

Buffett doesn't see signs that a bubble is forming in bonds or any other assets. But he says it's unusual that rates have remained low this long.