Egyptian authorities have blocked an encrypted messaging app routinely used by journalists and activists.

Many Egyptian users began reporting serious connectivity issues with Signal, leading the United States-based owner Open Whisper Systems to investigate the complaints. On Monday evening, the company confirmed via Twitter that "Egypt is censoring access to Signal."

Open Whisper Systems also said that it will start "deploying censorship circumvention" for the app in the next few weeks. It suggested users turn to Tor or a VPN in the meantime.

Egyptian officials have not publicly confirmed that the app has been blocked, but the Egyptian security agencies are known to monitor other services such as Facebook and Facebook Messenger. These were formerly used heavily by activists to plan protests and organize events.

A report in The Intercept earlier this year said Egyptian activists have turned to Signal for communication. The apps encryption makes it much more difficult for security agencies to monitor than other services.

In addition to text messages, Signal users can make and receive calls through the app. The app's high-level of encryption has even been sought out by Facebook to use in its Whatsapp messaging app.

Some Egyptian social media users also said that access to Telegram, another encrypted messaging app, has been unstable in recent days.

Activists and journalists have seen a widespread crack down on their activities in the last couple of years. A recent report by the Committee to Protect Journalist ranked Egypt third in the world for the number of imprisoned journalists. 

Human rights groups have been highly critical of the Egyptian government's crackdown, as many have been deported, imprisoned, tortured or even disappeared.