Amazon has been warned about the presence of pornographic content on its Kindle app that could be accessed by children.

Apple and Google said they had raised concerns with their fellow tech company, as both said such sexually explicit material risked falling foul of policy rules on their app stores.

It comes after the availability of adult content on the Kindle app, including books and comics featuring pictures of sex acts and naked women, was highlighted by Reuters news agency.

An Amazon statement said: "We're committed to providing a safe shopping and reading experience for our customers and their families, and we take matters like this seriously.

"We are reviewing all the available information and are taking action based on our findings."

Apple said it was working with Amazon to "ensure their app is compliant with our guidelines", while Google said it had "been in contact with the developer".

Reuters reported on the issue after being approached by two families who said their pre-teen sons had downloaded explicit material via Amazon's Kindle Unlimited subscription, which offers millions of titles for a monthly fee.

The boys viewed the content on the platform's iPhone app, which ranks highly on Apple's App Store.

Amazon does offer a Kids+ subscription, but the families told Reuters they signed up to Unlimited because it offered more age-appropriate books for their children to read.

However, it has no parental controls, meaning any user can access any of the books on offer.

How does sexual content end up on Kindle?

Amazon allows authors to self-publish through its Kindle Direct Publishing service.

It gets their work on to its platforms nearly instantaneously, providing quick access to the market's biggest player.

Recent estimates suggest Amazon controls two-thirds of the e-book market, well ahead of other providers - including Apple and Google, which have their own digital book stores.

Amazon's terms of use for self-publishing warn that "offensive or inappropriate" content could be refused, which may include anything that "contains pornography".

The company also uses software to help detect some banned content prior to publication.

What will Amazon do in response?

Amazon has changed the age rating of the Kindle iPhone app from four years and over to 12.

On the Google Play Store, it is now rated as "teen".

Amazon also noted that its terms require parental consent for users under 18.

The company told Reuters it would also update the Kindle app, but did not offer specifics.