U.S. Internet retailer Amazon.com Inc said on Thursday that its Kindle electronic book reader posted its best sales yet in the month of December, as the battle for the digital reader market heats up.

Without giving figures or comparisons and barely halfway through the month, the online retailer said that the Kindle was the most wanted product across the company during the holiday gift season of December.

Amazon sees the Kindle as a pivotal growth driver and is locked in a heated fight with rivals Sony Corp and Barnes & Noble over who will emerge with the best-selling electronic reading device.

Amazon on Thursday announced it would sell its product with free expedited shipping -- a contrast with its competitor Barnes & Noble, which has suffered delays in getting its Nook electronic reader into customers' hands.

Amazon's statement on the Kindle's success follows news on Thursday that its rival Sony Corp struck a deal with News Corp to make The Wall Street Journal and other publications available on its electric reader.

Shares of Seattle-based Amazon closed down 1 percent at $126.91 on Thursday on the Nasdaq.

Kindle