E-commerce giant Amazon has kept its cards close to its chest about the number of devices which feature its Alexa voice assistant that have been sold - up until this week when it finally broke its silence.

The Drum reports that the grand total to date sits at 100 million worldwide sales, which covers a wide range of products, including its own Echo line-up of smart speakers.

It is worth noting that this figure does not cover Amazon-branded products alone but includes thousands of other third-party gadgets that are compatible as well.

The actual number of people using Alexa on a regular basis remains a mystery, and earlier indications about the effectiveness of the service as a tool for safe shopping online have been relatively bleak.

Amazon itself confirmed last year that just two out of every 100 users of Alexa had ordered something just by asking their device of choice. Of this small group, only a tenth then went on to buy something else using the same method.

The suggestion is that while voice search functionality seems to be winning over the public, there is a lot less demand for the e-commerce features that are also afforded by the system.

Part of the problem may be that most shoppers prefer to see the item they are ordering before they commit. With Alexa, there is no immediate way to showcase images of potential purchases, especially if a user is restricted to engaging via a smart speaker.

As Alexa is added to more electronic items, this issue may gradually be dealt with, particularly if smart TVs embrace it on a wider scale. For the time being, Amazon will have to rely on smartphone and desktop users visiting its site and buying from its app to rake in billions each year.