Updated at 6:12 p.m. ET

By James Rogers, TheStreet

Amazon (AMZN) clearly wants to reassure investors that its Kindle e-book reader is coping with the threat posed by Apple's (AAPL) iPad, but actual Kindle sales numbers are still hard to come by.

In a statement released today, Amazon said the latest versions of the Kindle, launched in late July, are the fastest-selling Kindles ever. The Amazon.com and Amazon.co.uk websites have sold more of the new Kindle and Kindle 3G than any other products during the last four weeks, according to the online retail giant.

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Amazon's true Kindle performance, however, remains clouded in secrecy. Despite plenty of hype over the last few years, the retailer has not released any actual sales figures for its e-book device, although a company spokeswoman told TheStreet in an e-mail today that Amazon has sold "millions of Kindles overall."

Last month, when Amazon released its second-quarter numbers, the retailer said the Amazon.com site is selling more Kindle books than hardcover books. The U.S. Kindle Store now has more than 630,000 books available, it added.

"Kindle device unit sales accelerated each month in the second quarter -- month-over-month, and year-over-year, and that's just counting online sales," added the Amazon spokeswoman.

Amazon nonetheless faces stiff competition in the increasingly crowded e-book market. In addition to Barnes & Noble's (BKS) Nook, the prospect of an imminent Google (GOOG) tablet launch should also be cause for concern. Like the Apple iPad, the Google tablet will likely serve a number of functions, from e-book reader to gaming capabilities.

Apple, unlike Amazon, has been happy to reveal its unit sales; Apple said it has sold more than 3 million iPads since the device was launched in early April.

With competition intensifying, Amazon has repeatedly slashed its Kindle pricing. When the original Kindle launched in 2007, it was priced at $399, and by last year the price dropped 35% to $259. The new Kindle costs $189 for a 3G and Wi-Fi version while a Wi-Fi only Kindle is available for just $139.

Investors surveyed by TheStreet earlier this month predicted that Amazon will be forced to cut its Kindle prices even further. Some 48.7% of voters said the Kindle's price will bottom out at $99 and another 31.8% think it could go even lower.

Amazon shares rose 1.9%, to $126.85 today, outpacing the modest uptick in tech stocks.

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