Amazon.com has reported disappointing sales in the holiday quarter and said revenue in the current period may miss estimates, raising concerns that rising spending on warehouses, movies and gadgets isn't yet translating into faster growth.

Revenue increased 22 per cent to $US43.7 billion ($57.03 billion) in the fourth quarter, the Seattle-based company said on Thursday in a statement. Analysts estimated $US44.7 billion, according to data compiled by Bloomberg. Sales in the current quarter will be $US33.3 billion to $US35.8 billion, the company said, compared with analysts' projections of $US36 billion.

Foreign currency changes reduced fourth-quarter sales growth by 2 percentage points, the company said. In Amazon Web Services, the company's cloud-services division, revenue was $US3.5 billion, up 47 per cent from a year earlier.

While cloud computing is Amazon's fastest-growing and most profitable segment, that's down from a 69 per cent rise in the third quarter. Price reductions from the unit contributed to the slowdown, said Ron Josey, an analyst at JMP Securities.

"Expectations got way ahead of themselves in the fourth quarter and this is a reset," he said.

Shares fell as much as 4.6 per cent in extended trading after closing at $US839.95. The stock had gained more than 10 per cent over the past month in anticipation of a strong holiday period.

Retail competitors, including Wal-Mart Stores, are struggling to match Amazon's quick delivery on a wide assortment of goods as shoppers shift their spending from stores to websites and smartphones.

US online sales in November and December totalled $US91.7 billion, up 11 per cent from the previous year, according to Adobe Systems. The world's largest online retailer also attracts customers with its $US99-a-year Amazon Prime subscription, which includes delivery discounts, music and video streaming and photo storage.

Spending surge

Amazon reported operating expenses rose 23 per cent to $US42.5 billion in the quarter. The spending included $US5.7 billion to store and deliver items, particularly those ordered with fast shipping by Prime customers.

The company's forecast expects less profit than a year ago even though revenue is increasing, which is why investors are concerned about spending, said Michael Pachter, an analyst at Wedbush Securities.

"It means they're going to spend a ton of money," Pachter said. "When you see revenue go up and earnings go down, it spooks people."

"Tens of millions" of new members joined Prime last year, chief executive officer Jeff Bezos said in the statement. Prime had an estimated 65 million US members at the end of September, according to Consumer Intelligence Research Partners in Chicago. The company doesn't disclose the number of Prime members.

The company's spending on logistics show no signs of slowing. Amazon announced Monday it would build a $US1.49 billion air hub near Cincinnati to accommodate its growing fleet of cargo planes. The hub and planes make Amazon less reliant on United Parcel Service and FedEx to quickly reach customers, and complement its network of warehouses around the country.

Holiday expectations

Expectations for holiday revenue were too high given the company's growing dominance in e-commerce, said Vic Anthony, an analyst at Aegis Capital Corp.

"Amazon stock tends to get bid up over the euphoria of retail shifting online during the holiday shopping season, and now it's taking a little breather," he said.

Net income was $US749 million, or $US1.54 a share, from $US482 million, or $US1 a share, a year earlier. Analysts estimated profit of $US1.36 a share.