Iranian users of the Signal messaging service said Monday they have lost access to the app on their mobile devices.
Users in several neighbourhoods in the capital Tehran and at least three other major cities contacted by The Associated Press said access to the app was blocked gradually beginning in the afternoon.
They said they could use the system through virtual private networks, services that shield internet users by encrypting their data traffic. Some still had access through the desktop version of the app.
State media did not report that the app was blocked and calls to authorities were not immediately returned Monday.
It's not clear how many Signal users there are in Iran but many migrated to the app after Whatsapp updated its privacy policy in January.
The Iranian government has long blocked access to many websites and social media platforms, from YouTube and Facebook to Twitter and Telegram.
Last week, Iran's judiciary released the country's telecom minister, Mohammad Javad Azari Jahromi, on bail after he was summoned for prosecution. Officials cited his refusal to block Instagram and other foreign social media and messaging systems.
Many Iranians, mostly youth, access social media through VPNs and proxies. Instagram and WhatsApp remain unblocked.
Emerging hard-liners in the country's parliament and other powerful bodies view social messaging services as part of “soft war” by the West against the Islamic Republic.