Amazon.com reported a jump in third-quarter profit and sales, beating analyst estimates. But the Seattle-based online retailer's operating expenses rose sharply and the company's shares were down about four percent in after hours trading.

Amazon said Q3 net income increased 16 percent to $231 million, or 51 cents per diluted share, compared with net income of $199 million, or 45 cents per share, the same quarter a year ago. The company's Q3 net sales jumped 39 percent to $7.56 billion from the year ago quarter.

Analysts had estimated third quarter earnings of 48 cents a share on revenue of $7.36 billion.

Operating expenses for the third quarter were up more than 40 percent to $7.29 billion, as the company continued to add infrastructure capacity around its e-commerce and cloud-computing businesses. Amazon shares were down about four percent in after-hours trading to $159.12.

Amazon gave guidance for its fourth quarter, saying it expects sales of between $12 billion and $13.3 billion, representing growth of between 26 percent and 40 percent from the same quarter a year ago. It expects Q4 operating income of between $360 million and $560 million, which ranges from a 24 percent decline to 18 percent growth from the year-ago quarter.

The electronics and general merchandise category was strong for Amazon in the third quarter, growing 68 percent worldwide to to $3.97 billion. The media category — books, CDs and DVD/Blu-ray discs — grew at a more modest 14 percent to $3.35 billion.

Amazon increased its spending on infrastructure and marketing in the third quarter and indicated that trend will continue.

Fulfillment costs in Q3 were up nearly 46 percent from the year-ago quarter, to $680 million. In an earnings conference call, Amazon Chief Financial Officer Tom Skzutak said the company has opened 10 fulfillment centers this year with plans for several more in the coming weeks ahead of the peak holiday shopping period. Amazon uses fulfillment centers to store and ship orders for its own e-commerce operation and for many third-party businesses that sell through Amazon.

"We are adding additional capacity in Q4 from a fulfillment and infrastructure standpoint," Skzutak said. "If we continue to grow like we're growing you should expect we'll add capacity next year."

Marketing costs in Q4 rose 61 percent to $241 million, in part due to TV advertising for Amazon's Kindle reader and marketing support for online shoe retailer Zappos, which Amazon acquired last year.

Amazon said its latest Kindles are the "fastest-selling Kindles of all time and the bestselling products on Amazon.com and Amazon.co.uk," but as is customary the company gave no concrete sales figures.

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