Signal appears to have been blocked in China, where the messaging app, one of a few foreign platforms still accessible within the Great Firewall, was often used by dissidents, activists and journalists.

Users of the app in Beijing and other Chinese cities said that they were unable to send messages on the platform beginning on Monday evening and could only access the app while using a virtual private network, which masks one’s location.

“Signal has been walled,” users wrote on the microblogging site Weibo, referring to the country’s sophisticated Internet controls barring access to Google, Twitter, Facebook, foreign news sites, and messaging apps such as WhatsApp and Telegram.

Signal’s website also appeared to have been blocked in China as of Tuesday, according to Greatfire.org, which monitors censorship in the country. It was unclear whether the blocking was permanent or a temporary outage. The California-based company did not respond to a request for comment.

Faced with surveillance on Chinese apps, Chinese human rights lawyers, activists and other advocates had turned to Signal to exchange information about cases, talk to journalists and communicate with dissidents abroad. Downloads of Signal in China saw an uptick in January after Tesla CEO Elon Musk encouraged his followers to download the app.

The apparent blocking of Signal comes as China’s top leaders exert more control over the Internet. At an annual legislative session this month, Premier Li Keqiang called for “strengthening the content creation and management” of the Internet to develop “a positive and healthy online culture.”

Chinese Internet companies, platforms, bloggers and ordinary users have come under more scrutiny. Authorities have reportedly ordered Internet giant Alibaba to sell its media assets, including stakes in Weibo and the South China Morning Post newspaper, out of concern over the company’s ability to influence public opinion.

Last month, the Cyberspace Administration of China ordered bloggers and other content creators known as “self media” to apply for government-approved credentials before publishing. Several Chinese citizens were detained in February for comments made online “defaming” Chinese soldiers who died in a border clash with Indian forces last year.

Signal is the latest foreign platform to be subjected to China’s censors, a sign authorities may be moving more quickly to cut off access to unregulated platforms.

The audio app Clubhouse was blocked within a week of it taking off among Chinese users within and outside of the country, who used the platform to debate sensitive topics such as human rights allegations in Xinjiang, protests in Hong Kong or the 1989 Tiananmen Square massacre.