Amazon.com Inc forecast a lower-than-expected profit for its second quarter, raising concerns about the online retailer's future margins and sending its shares down nearly 6 percent.

Investors had hoped the momentum behind Amazon's blow-out results in the last two quarters would continue through the year. But Amazon, like rival eBay Inc , disappointed those who sought greater acceleration in earnings growth as consumer confidence picks up.

The fact that operating income (outlook) has a much lower range to it is the key driver of the downside here after the close, said Hudson Square Research analyst Scott Tilghman.

In this case what we have is a very solid first quarter, but it looks like margins compressed ... so that might be a concern going through the rest of the year, he said.

Amazon on Thursday estimated operating profit in its second quarter of $220 million to $320 million on revenue of $6.1 billion to $6.7 billion.

Analysts, on average, have been expecting an operating profit of $327.8 million on revenue of $6.43 billion, according to Thomson Reuters I/B/E/S.

Colin Gillis, an analyst at BGC Partners, noted that the clout Amazon enjoyed during the recession with its suppliers by buying in bulk is likely to lesson amid economic recovery.

The cost of revenue is likely to pick up. They are not able to make as much money on the backs of their suppliers, said Gillis.

But others noted that the focus on potentially weak operating margins was misguided, as Amazon routinely exceeds its outlook.

Because the stock traded down in the after-market it's easy to jump to the conclusion that there is something wrong, said Benchmark Co analyst Frederick Moran, calling the results very strong.

Amazon's history with guidance is extremely cautious and extremely conservative and the bottom end of the range is never anything but misleading, he added.

Amazon reported net income for the first quarter of $299 million, or 66 cents per share, up from $177 million, or 41 cents per share, a year earlier. Analysts, on average, had been expecting earnings of 61 cents per share.

Revenue jumped 46 percent to $7.13 billion -- above the $6.87 billion expected by analysts.

Shares of Amazon fell to $141 after hours from a close of $150.09 on Nasdaq. They are up 90 percent in the last year and have gained nearly 12 percent year to date, hitting a new high of $151.09 on Thursday before it released the results.

IPAD CHALLENGE

As Amazon has grown, adding new categories and more third-party vendors, the company has spent more on such items as a new distribution center in Canada, corporate office space in Seattle and fresh hires.

Operating expenses amounted to 77 percent of net sales in the first quarter versus 76.5 percent a year ago.

Marketing costs rose to $201 million, representing 2.7 percent of revenue, up from 2.5 percent in the year-ago period, Amazon said. The company has been advertising its Kindle electronic reader in television and print ads.

But it has also seen revenue surge, by 47 percent in North America and 45 percent internationally during the first quarter, with gains from currency translations helping results.

Amazon's Kindle has kept the company in the limelight, but now faces a market share battle against Apple Inc's recently introduced iPad.

Asked about competition from Apple, Chief Financial Officer Tom Szkutak said demand for Kindle remains very strong. He added: We're not breaking out any precise numbers on that.

Amazon's media segment, which includes books, has been improving, as it strives to offer the lowest prices against fierce competition from rivals such as Wal-Mart Stores Inc's online unit. That segment rose 26 percent in the first quarter.

Earlier this week, Target Corp announced that it would begin selling the Kindle in its stores, marking the first time that Amazon has allowed distribution of its wares outside its own online channel. The iPad is sold at Best Buy stores as well as at Apple's own stores.

(Additional reporting by Phil Wahba, Lisa Baertlein and Dhanya Skariachan; Editing by Michele Gershberg and Richard Chang)

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