Luxembourg's privacy regulator has asked Amazon for information regarding its Alexa voice assistant, an indication of rising regulatory unease over companies' use of personal data.

The increasing popularity of Alexa, Apple's Siri and Google Assistant has triggered concerns from politicians and privacy enforcers over how some companies handle recordings from users interacting with their voice assistants.

Apple and Google last week halted reviews of recordings. The Guardian said Apple's contractors tasked with the job regularly heard confidential information and private conversations.

Luxembourg, which is the lead privacy watchdog for Amazon where it has its European headquarters, said on Thursday that it is in touch with the U.S. online retailer regarding Alexa.

"At this stage, we cannot comment further about this case as we are bound by the obligation of professional secrecy," a spokesman said.

Amazon said it has taken steps to resolve any concerns.

"For Alexa, we already offer customers the ability to opt out of having their voice recordings used to help develop new Alexa features," a spokeswoman said.

"The voice recordings from customers who use this opt-out are also excluded from our supervised learning workflows that involve manual review of an extremely small sample of Alexa requests."