Amazon's Kindle may face a new rival in Apple's freshly unveiled iPad tablet computer but for now the online retail giant is enjoying its reign as king of the electronic book reader market.

The Seattle, Washington-based Amazon said Thursday that strong holiday sales sent net profit soaring 71 percent during the fourth quarter of the year to 384 million dollars while sales rose 42 percent to 9.52 billion dollars.

Earnings per share of 85 cents were better than the 72 cents expected by Wall Street analysts.

For the year, net profit rose 40 percent to 902 million dollars while sales increased 28 percent to 24.51 billion dollars, Amazon said.

Amazon did not release a figure for Kindle sales during the fourth quarter but the popular e-reader was the only item singled out in a statement by Amazon founder and chief executive Jeff Bezos.

"Millions of people now own Kindles," Bezos said. "And Kindle owners read, a lot. When we have both editions, we sell six Kindle books for every 10 physical books."

Bezos said the ratio "includes only paid books - free Kindle books would make the number even higher."

Amazon said the US Kindle store had increased its number of books to 410,000 including 100 of 112 New York Times bestsellers.

Amazon declines to reveal sales figures for the Kindle but Bezos said in October that the device is the company's "number one bestselling item by both unit sales and dollars" across all product categories.

Forrester Research estimates the Kindle has a nearly 60 percent share of the US market followed by the Sony Reader with 35 percent and expects e-reader sales to double to six million units this year.

Those estimates came out before Apple plunged into the e-reader market with the release on Wednesday of its iPad tablet and the launch of an "iBookstore" for the iPad along the lines of its iTunes music store but offering digital books, newspapers and magazines.

The iPad offers a color touchscreen as opposed to the black-and-white Kindle but also carries a much heftier price tag - 499 dollars for the cheapest iPad as opposed to 259 dollars for the basic Kindle.

Amazon said North America sales rose 36 percent in the quarter to 4.96 billion dollars while international sales from German, Japanese, French and Chinese sites were up 49 percent to 4.56 billion dollars.

Amazon said it expected sales of between 6.45 billion dollars to 7.0 billion dollars in the current quarter.

Amazon also said the board of directors had authorized the repurchase of up to two billion dollars worth of the company's common stock.

Amazon shares gained 2.67 percent on Wall Street on Thursday to close at 126.03 dollars and were trading another two percent higher in after-hours electronic trading.