Amazon has grown sales to almost $53 billion and posted better-than-expected profits in the second quarter of its financial year.

The tech giant grew net sales by 39 per cent to $52.9 billion for the three months to 30 June 2018. Operating income increased to $3 billion from $628 million in the second quarter of 2017.

Founder and CEO Jeff Bezos used the quarterly update to highlight Amazon's digital assistant, Alexa.

"We want customers to be able to use Alexa wherever they are," he said.

"There are now tens of thousands of developers across more than 150 countries building new devices using the Alexa Voice Service, and the number of Alexa-enabled devices has more than tripled in the past year.

"Our partners are creating a wide variety of new Alexa-enabled devices and experiences, including soundbars from Polk and Sonos; headphones from Jabra; smart home devices from ecobee and First Alert; Windows 10 PCs from Acer, HP, and Lenovo; and cars from automakers including BMW, Ford, and Toyota."

Amazon expects net sales for the third quarter of 2018 to be between $54 billion and $57.5 billion.

Operating income is expected to be between $1.4 billion and $2.4 billion, compared with $347 million in third quarter 2017.