Amazon.com Inc. posted quarterly net profit on Tuesday that more than tripled, surprising Wall Street, as U.S. sales jumped, sending shares up 17 percent after-hours.

The company also raised its forecast for full-year sales.

It's a blow-out second quarter -- it's an amazing performance, said Bernstein Research analyst Jeffrey Lindsay. A lot of it (growth) seems to have come in the United States, which is particularly surprising since that's considered Amazon's most mature market.

Net income in the second quarter rose to $78 million, or 19 cents per share, compared with $22 million, or 5 cents a share, a year earlier. Revenue rose 35 percent to $2.89 billion in the second quarter, which is seasonally the slowest. In North America, net sales rose 38 percent, Amazon said.

Earnings of 19 cents per share topped the Wall Street target of 16 cents, and revenue also beat the $2.8 billion average expectation, according to Reuters Estimates.

Lindsay said electronics and general merchandise posted an impressive 55 percent growth, which he called unusually good. He said results from North America were the best he had seen in several years. The company said North American growth was the highest in more than six years.

Chief Financial Officer Tom Szkutak attributed part of the quarterly sales growth to Amazon Prime, the discount shipping program that charges customers $79 annually for unlimited two-day shipping.

Subscribers are increasing, customers are buying more, it's becoming a more meaningful part of our overall units. It's certainly impacting the growth you're seeing this quarter, Szkutak told reporters during a conference call.

Operating profit margins, a source of much worry on Wall Street in recent years due to high investment spending, came in at 4 percent of worldwide sales in the quarter, well above the 2.2 percent seen a year earlier, when spending on technology and content was significantly higher.

RAISING FULL-YEAR SALES FORECAST

For the third quarter, the company said it expects net sales in a range of $3.0 billion to $3.18 billion, with operating income in a range of $75 million to $110 million.

Analysts, on average, have been expecting third-quarter revenue of $3.01 billion, according to Reuters Estimates.

Amazon raised its full-year sales forecast to a range of $13.8 billion to $14.3 billion. Previously, the company said it expected a range of $13.4 billion to $14.0 billion.

Amazon shares have surged since last quarter's results showed improved profit margins and pared-back spending. Bullish investors, jumping on signals that investments in technology and content might be beginning to pay off, have pushed Amazon's price up nearly 55 percent as of Tuesday's close since first-quarter earnings were announced in April. Shares have more than doubled since year-ago results came out.

Amazon shares rose 17.1 percent to $81.09 from a close of $69.25 on Nasdaq.

The company is now valued well above major Internet players as well as brick-and-mortar retailers. Amazon shares, as of Monday's close, traded at 52 times 2008 expected earnings, compared with 21 times at eBay Inc., 46 times at Yahoo Inc. and 14 times at Wal-Mart Stores Inc.

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