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The online retailer, Amazon, has reported a 16% rise in third-quarter profit, as its Kindle e-book reader continues to bolster sales.

Amazon made a net profit of $231m (£147m) in the three months to September, up from the $199m it made in the same period a year earlier.

Revenues rose 39% to $7.56bn.

But the firm also said that its total operating expenses rose more than 40% to $7.29bn, sending its shares down 5% in after-hours trading.

The Kindle launched in the US in 2007. Since going on sale in other countries, including the UK, in 2009, it has become Amazon's fastest-selling and best-selling item.

Revenue from Amazon's largest sales category - electronics and other general merchandise, which includes the Kindle - shot up 68% to $3.97bn.

Revenue from books, CDs, DVDs and other media grew by 14% to $3.35bn.

Looking ahead, Amazon said it expected sales in the last three months of the year to be between $12bn and $13.3bn, representing growth of between 26% and 40%.

Mike Koskuba, an analyst at Victory Capital Management, said he was confident the retailer would continue to perform well.

"I do believe the holiday [season] is going to be really quite strong for Amazon. Kindles are going to sell exceptionally well," he said. "I do think that general merchandise will pick up as well in the fourth quarter."