The ban, which began Monday, comes after Telegram refused to hand over encryption keys that would give the Kremlin access to private messages on its platform. The Russian government had demanded access to the service to track terrorist threats.

Durov, however, warns that banning Telegram could actually compromise Russian state security and the security of 15 million Russian Telegram users. Worldwide, Telegram has about 200 million users.

“Russia’s national security will decrease, as some of the personal data of Russian citizens will pass from [Telegram] to U.S.-controlled WhatsApp/Facebook,” he wrote in a post on Russian social network, VKontake, a network that he founded.

Kremlin officials are also known to use Telegram, with Kremlin spokesperson Dmitry Peskov telling journalists Friday that his office would switch from Telegram to another platform known as ICQ, Reuters reports. ICQ is owned by billionaire Usmanov’s internet portal Mail.ru, which also owns VKontake.

Durov, a Russian-native living in self-imposed exile, has vocally fought Russia’s privacy policies. He says he still plans to fight the ban, saying Telegram would seek “built-in” methods to bypass the firewall.

Telegram meanwhile recently completed another round of funding, raising a total of about $1.7 billion.