Online retailer Amazon.com Inc. on Wednesday reported a 31 percent jump in revenues in the fourth quarter, but it's profits failed to meet Wall Street expectations and its stock fell by more than 13 percent in after-hours trading.

The Seattle company said net income for the quarter ended Dec. 31 was $347 million, or 82 cents a share, compared with profits of $73 million, or 17 cents a share, for the same period a year ago. Net income for the latest quarter, however, included a $244 million tax benefit.

Excluding the tax benefit, profits were $149 million, or 35 cents a share, which fell below analysts' average estimate of 40 cents a share, according to Thomson First Call. Amazon.com's stock fell 13.68 percent, or $5.73, in after-hours trading to $36.15.

Revenues for the quarter rose to $2.54 billion from $1.95 billion a year ago.

In the last couple of years, Amazon.com has come under increasing pressure from major brick-and-mortar retailers, such as Wal-Mart Stores Inc., who are increasingly leveraging the web to boost sales. To counter the tougher competition, Amazon.com has offered promotions, such as free shipping on orders over $25, which have cut into profits and have worried some analysts.

Earlier Wednesday, the company announced an express-shipping membership program that offers shoppers unlimited two-day shipping on more than a million in-stock items for $79 a year. Members could also choose overnight shipping for $3.99 per item.

The company normally charges $9.48 for two-day shipping on a single book and $16.48 for overnight delivery. The membership program works across books, DVDs, music, electronics, kitchen items, tools, health and personal care products, and more.

"What Amazon.com is trying to do here is drive loyalty and hold on to customers," Heather Dougherty, retail analyst for Nielsen/NetRatings, said. "It's really more of a protective measure for them."

Defensive measures, however, cost money. Amazon.com on Wednesday said operating income for the first quarter of this year would be between $80 million and $110 million, or a decline of between 28 percent and 0 percent, compared with the same period a year ago.

In a letter to customers published on its website, Jeff Bezos, founder and chief executive of Amazon.com, said the new Amazon Prime shipping program would be "expensive for Amazon.com in the short term."

"In the long term, we hope to earn even more of your business, which will make it good for us too," Bezos said.

Also in its first-quarter forecast, Amazon.com predicted net sales between $1.8 billion and $1.95 billion, a year-to-year increase of between 18 percent and 27 percent.

For the full year, net sales were expected to increase from 16 percent to 25 percent to between $8.05 billion and $8.65 billion. Operating income was expected to be between $385 million and $510 million, or between a 13 percent decline and 16 percent growth.