It appears encrypted messaging app Signal is being blocked by China, with users needing to use a VPN to connect.

Signal is one of the most popular encrypted messaging apps, and widely considered to be the most secure. The US Senate, the EU Commission and some US military units recommend their members use the app, thanks to the level of security it offers.

According to a report in The Washington Post, it appears China is actively blocking the app, with users reporting they need to use a VPN to be able to access the service. China has a well-documented history of blocking and censoring various apps and services, so much so that “The Great Firewall of China” has come to refer to China’s efforts in this regard. Google, Twitter and Facebook are similarly blocked.

Zhao Lijian, a spokesman for China’s Ministry of Foreign Affairs, denied the reports, saying he was “not aware of the situation,” according to WP.

“What I can tell you is that as a principle, China’s internet is open, and the Chinese government manages internet-related affairs according to law and regulations,” Zhao said.

Signal has yet to address the situation publicly.