Apple has pulled popular encrypted messenger app Telegram from the App Store, following reports of "inappropriate content" hosted on the platform that violated Apple's developer guidelines for iOS apps.

Users first noted Telegram's absence late on Wednesday via Reddit, after App Store searches for the app began returning results for Viber, Skype, and Messenger instead. Both the original app and experimental offshoot Telegram X still don't appear on the App Store at the time of writing.

The exact reason for the apps' removal isn't yet clear, but Telegram founder Pavel Durov responded to a tweet on Thursday by explaining that the company was alerted by Apple to "inappropriate content" made available to users of both apps, which led to them being pulled.

We were alerted by Apple that inappropriate content was made available to our users and both apps were taken off the App Store. Once we have protections in place we expect the apps to be back on the App Store. — Pavel Durov (@durov) February 1, 2018

• A method for filtering objectionable material from being posted to the app

• A mechanism to report offensive content and timely responses to concerns

• The ability to block abusive users from the service

• Published contact information so users can easily reach you Apps with user-generated content or services that end up being used primarily for pornographic content, objectification of real people (e.g. “hot-or-not” voting), making physical threats, or bullying do not belong on the App Store and may be removed without notice.

Apple's review guidelines include a section on user safety that prohibits "upsetting or offensive content", while a sub-section covering user-generated content requires that the relevant apps include:

We'll update this post if we hear more details on the situation.

Update: At around 8:00 a.m. PT, company CEO Pavel Durov tweeted that Telegram is now back on the iOS App Store.