Amazon staff listen to recordings of customer interactions with voice-based assistant Alexa to help train the artificial intelligence's responses, it has been reported.

The tech giant uses review teams around the world to analyse and annotate some recordings gathered from Amazon Echo smart speaker devices, with the interactions then used to improve the programming of the Alexa software, according to Bloomberg.

The Alexa companion app, which is used to set up smart devices and manage settings linked to the assistant, keeps a log of interactions between users and the assistant, and can be listened to or deleted from within the app.

The report claimed staff had on occasions reported hearing recordings they described as distressing, but Amazon said it had procedures in place for workers to follow when they heard such audio, including sharing via an internal chat room to relieve stress.

It also said these chat rooms were used to help identify muddled words, as well as to share amusing recordings.

In a statement, Amazon confirmed that it did use some recordings as part of its work to improve Alexa's ability to understand human language and speech patterns, but had strict security systems in place to keep user data safe.

"We take the security and privacy of our customers' personal information seriously. We only annotate an extremely small sample of Alexa voice recordings in order improve the customer experience," said a spokeswoman.

"For example, this information helps us train our speech recognition and natural language understanding systems, so Alexa can better understand your requests, and ensure the service works well for everyone.

"We have strict technical and operational safeguards, and have a zero-tolerance policy for the abuse of our system.

"Employees do not have direct access to information that can identify the person or account as part of this workflow.

"All information is treated with high confidentiality and we use multi-factor authentication to restrict access, service encryption, and audits of our control environment to protect it."

The help page of the Amazon website dedicated to Alexa states that Amazon may "use your requests to Alexa to train our speech recognition and natural language understanding systems", explaining that the more data used to train these systems, the better the voice assistant is able to work.

It also confirms that the company "associate your requests with your Amazon account to allow you to review your voice recordings, access other Amazon services ... and to provide you with a more personalised experience".

Concerns have been raised by some in the past that smart speaker systems could be used to constantly listen in to user conversations, often with the aim of targeting users with relevant advertising.

Last year, Facebook boss Mark Zuckerberg was asked by a committee of the US Senate if the social network used microphones in mobile devices to listen in on people, something he strongly denied, labelling it a "conspiracy theory".

The popularity of smart speakers has grown exponentially since they were first introduced to the UK in 2016.

According to research from last year, one in 10 people has one or more smart speakers in their home, with Amazon's range of Echo smart devices the market leader.