Amazon has admitted it employs temporary workers, sometimes working from home, to transcribe the voice commands of its Alexa virtual assistant.

The acknowledgment follows revelations by the Welt am Sonntag newspaper that workers hired by an agency in Poland were allowed to hear the voice recordings of its German users.

The disclosure heightens privacy concerns amid the growing popularity of interactive devices that allow users to give voice instructions to make their gadget play music, turn on lights or check the news and weather.

Read more: Protecting children from voice assistants' data collection zeal

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Accidental recordings

Previous revelations have shown how the voice assistants can be triggered unintentionally to record conversations.

Last week, Britain's Guardian reported that contractors working on Apple's virtual assistant, Siri, had heard more than just users' instructions, having listened in to events including people making drug deals and having sex.

Welt’s article, meanwhile, revealed how the voice commands given by Alexa's German users are not only heard by Amazon employers but also by agency workers in Poland.

The newspaper reported that contractors, recruited by the agency Randstad, were allowed to work from home or on the road, a practice that potentially opens up users' personal information to be copied or shared without scrutiny.

Read more: When smart devices pass secrets to the police

More than 100 million devices with Alexa on board have been sold, including smart speakers, tablets and home appliances

Teleworking jobs offered

Welt said that while the tech giant had insisted that only trusted staff had access to the recordings, job advertisements by the agency offered workers with strong German language skills the promise of "teleworking throughout the country," after being trained by Amazon at their Gdansk office.

Amazon admitted that audio transcription can be carried out at home but insisted "there are strict security measures and policies that every employee must adhere to." For example, working in public places is prohibited, it said.

One agency worker told Welt it was possible to hear names or places in the voice recordings, which could potentially allow Alexa users to be identified.

Read more: Sexy Siri, you made a fool of everyone

Security tightened

On Saturday, Amazon announced a new global privacy feature allowing users to explicitly block their voice commands from being reviewed by humans.

Apple and Google also recently suspended human audits of voice assistant recordings following similar concerns.

German and US regulators have begun investigations into the possible violation of privacy by the tech giants.

Although recent cases may give consumers the impression that someone is “listening" to their conversations, it is rarely true. Only a tiny proportion of the voice recordings are ever replayed by the tech giants, who insist the monitoring is only done to improve services.

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