China appears to have blocked the encrypted messaging app Signal, which was frequently used by journalists, activists and dissidents as an alternative to heavily surveilled local social media channels.

Although the app used to be accessible to anyone, it now can only be used with a virtual private network (VPN), according to users in the country. Without it, messages currently cannot send and calls won’t go through. Signal’s website was also inaccessible in mainland China on Tuesday.

Signal is the country’s latest foreign social media casualty. The country’s so-called “Great Firewall” already blocks sites such as Facebook, Google, Twitter and a host of foreign news outlets, as well as other chat apps like Telegram and WhatsApp.

Last month, the Clubhouse app was also blocked in the country after Chinese users began participating in live chats about topics seen by Beijing as politically sensitive, including the ongoing genocide in Xinjiang.

Signal’s end-to-end encryption made it possible for users to communicate without a third party viewing messages or listening in. On other apps such as Tencent’s WeChat, the country’s most dominant platform, users have been detained by authorities for messages they’ve sent in private and group chats. Certain terms and images are also wiped from the app, with posts deemed too politically sensitive taken down altogether.

Signal’s app and website still appear to be working normally in Hong Kong on Tuesday, and its app is still visible and available to download on Apple’s app store in the mainland.

China’s foreign ministry spokesman Zhao Lijian said Tuesday he was “not aware” of a Signal ban, stating that “as a principle, China’s internet is open.”

This is not the first time the app has gone down in the country without warning or reason. In past instances, service later resumed.

The app had been downloaded nearly 510,000 times onto Apple devices in China, according to data from Sensor Tower. It has been downloaded 100 million times globally off Apple’s app store and Google Play. In comparison, Tencent’s WeChat app, the country’s most popular messaging app, has more than a billion global users.

The crackdown on Signal hits as Beijing executes a broader crackdown on free speech and communications outside the reach of government censors.

News emerged Monday that authorities have reportedly ordered tech giant Alibaba to sell off its many media assets out of concern over its ability to sway public opinion. Last month, Beijing enacted its tightest restrictions to date on the publication of original online content via short video and blogging accounts.