The Signal encrypted messaging app appeared to no longer work in China without the use of a virtual private network (VPN) on Tuesday morning, users in the country said.

Chinese cyber authorities have become increasingly stringent in recent years, expanding the scope of banned apps, media and social media sites in the country.

Signal’s website was also inaccessible in China on Tuesday morning.

The app was still available on Apple’s App Store in China since Tuesday morning, and the app and website appeared to be working normally in Hong Kong.

The Signal press team did not immediately respond to a request for comment.

China’s cyberspace administration could not be reached immediately for comment.

The app was previously not available in third-party Android stores in China, where Google services are largely blocked.

On several occasions, the app had previously stopped working in China without a VPN for no given reason, only to resume use.

Signal saw an increase in downloads worldwide after January 6 when WhatsApp updated its privacy terms, reserving the right to share user data, including location and phone number, with its parent. Facebook Inc and units such as Instagram and Messenger.

Signal has been downloaded nearly 510,000 times on iOS in China, and has been downloaded 100 million times globally from the App Store and Google Play combined, according to data company Sensor Tower.

Tencent WeChat’s all-in-one mobile app is the dominant messaging app in China, with its payment and other service functions, has more than one billion users worldwide.