The third quarter "is generally a high investment period for the holiday", he said.
Analysts estimated operating profit of US$863.5m, and shares fell as much as 4.3 per cent to US$1001.80 in extended trading yesterday after the forecast and earnings were reported.
Amazon's stock has gained 39 per cent this year.
"Spending is always a concern with Amazon, but investors eventually give Amazon a pass because Amazon invests in growth opportunities," said Victor Anthony, an analyst at Aegis Capital Corp.
Amazon did not wait until the second half to pick up the pace of spending.
Second-quarter expenses increased 28 per cent to US$37.3b, the Seattle-based company said.
Investors have put increasing faith in chief executive Jeff Bezos to keep Amazon growing by entering new categories such as groceries and appliances, and expanding abroad. The company is moving into India and Australia, speeding up delivery times to as little as an hour on select products, adding new skills and devices for its voice-activated Alexa platform and producing more original movies and shows. Amazon hired more than 30,000 new employees "in the last few months", Bezos said.
Second-quarter sales gained 25 per cent to US$38b, topping analysts' projections of US$37.2b, according to data compiled by Bloomberg. Net income declined to US$197m, or US40c a share, from US$857m, or US$1.78 a share, a year earlier. Analysts estimated profit of US$1.42 a share, according to data compiled by Bloomberg.
Net sales in the third quarter will be US$39.25b to US$41.75b -- growth of 20 per cent to 28 per cent from a year earlier. Analysts estimated US$40b.
The forecast excludes Amazon's proposed US$13.7b deal for Whole Foods Market, where the company is betting it can replicate its online selling success. With 460 stores across the US, the acquisition is a major push into the US$800b grocery category dominated by Wal-Mart Stores, which has more than 26 per cent of the market. The up-market grocery-store chain this week reported improving results in the second quarter, with sales declining less than analysts expected.
Amazon dominates e-commerce in the US with its US$99-a-year Amazon Prime subscription, which includes delivery discounts, music and video streaming and is intended to keep shoppers engaged with the website. The company had 85 million Prime subscribers in the US as of June 30, an increase of 35 per cent from a year earlier, according to Consumer Intelligence Research Partners.
Amazon's subscription services revenue, mostly from Prime memberships, increased 51 per cent to US$2.17b in the second quarter. The company doesn't disclose the number of Prime subscribers.
Revenue from Amazon Web Services, its profitable cloud-computing division, increased 42 per cent to US$4.1b. The unit's sales increased 43 per cent in the previous quarter.
Sales of warehousing, packaging and other logistics services Amazon provides for e-commerce merchants increased 38 per cent to US$7b.
The numbers
• US$38b second-quarter sales, up 25 per cent
• US$37.3b expenses, up 28 per cent
• US$197m net income, down from US$857m