SAN FRANCISCO (AFP) - Online giant Amazon said on Thursday its profits more than tripled in the third quarter to US$252 million (S$351 million), but shares fell with the results that were weaker than expected.

Amazon extended its string of positive results after years of investments that produced little or no profit, helped by its cloud computing, streaming video and new products and services.

Profit surged from US$79 million in the same period a year ago, while revenues jumped 29 per cent to US$32.7 billion.

Amazon shares tumbled nearly five per cent on the results, which fell shy of most analyst forecasts.

Amazon founder and chief executive Jeff Bezos offered a cryptic comment on the results, referring to the company digital personal assistant Alexa, which responds to queries on its Echo speaker and other devices.

"Alexa may be Amazon's most loved invention yet - literally - with over 250,000 marriage proposals from customers and counting," said Bezos.

"And she's just getting better. Because Alexa's brain is in the cloud, we can easily and continuously add to her capabilities and make her more useful - wait until you see some of the surprises the team is working on now."