Mark Zuckerberg makes his keynote speech during Facebook Inc's annual F8 developers conference in San Jose - REUTERS

Facebook has been paying hundreds of people to listen to users' private audio clips without their knowledge.

The social media giant confirmed that, until very recently, it had employed human contractors to transcribed voice messages sent via its Messenger app in order to check the work of its AI systems.

It is the latest major tech firm to be caught listening to users' messages without their explicit knowledge, following similar revelations about Amazon, Google and Apple.

On Tuesday, a spokesman said the company had "paused" the practice "more than a week ago" after other companies' audio listening attracted scrutiny from the press.

Facebook's privacy policy does not explicitly mention that humans will listen to users' audio messages, only saying that it may share "content, communications and other information you provide" with "vendors and service providers who support our business".

It is not clear whether Facebook contractors listened to audio sent by users in Britain or in the EU.

The audio recordings were generated by Messenger's voice chat transcription option, which lets users send voice messages which are then transcribed by AI for the benefit of their recipient.

According to Bloomberg, which broke the news of the practice, workers at an outsourcing company called TaskUs listened to such clips in order to check whether the AI had transcribed them correctly.

But although the clips were anonymised, the workers were not told why they were transcribing them, and reportedly felt that their work might be unethical in light of Facebook's repeated denials that it is listening to users' conversations through their phones' microphones.

Those denials referred to a separate theory held by many users, who believe that Facebook is constantly recording their real-world conversations in order to target adverts at them. Mark Zuckerberg, Facebook's chief executive, has referred to that idea as a "conspiracy theory".