Online retail giant Amazon said Thursday that sales of digital books for the Kindle electronic reader have surpassed sales of print books.

"Customers are now choosing Kindle books more often than print books," Amazon founder and chief executive Jeff Bezos said in a statement.

"We had high hopes that this would happen eventually, but we never imagined it would happen this quickly," Bezos said. "We've been selling print books for 15 years and Kindle books for less than four years."

The Seattle, Washington-based Amazon said that since April 1, it was selling 105 Kindle e-books for every 100 print books, hardcover and paperback combined.

The company said it had sold more than three times as many Kindle books so far in 2011 as it did during the same period last year.

Amazon began selling print books in July 1995 and introduced the Kindle in November 2007.

The US Kindle store offers more than 950,000 books including 109 of the 111 New York Times best sellers.

Amazon does not release sales figures for the Kindle e-reader.