It’s no secret that Russia is up there with China when it comes to monitoring its citizens’ communications and blocking websites. According to TorrentFreak, the country’s government is increasing the pressure on walkie talkie app Zello by telling ISPs to block 15 million IP addresses, most of which belong to Amazon, as a way of taking it down.

Zello hit the headlines back in September when it topped the Apple app store charts after millions of new users joined in response to Hurricane Irma. The app simulates traditional two-way radios through the use of WiFi or cellular data, making it useful in search and rescue situations, and it now boasts over 100 million users worldwide.

Zello is yet another app to fall under the scrutiny of Russia’s Yarovaya law—a pair of federal bills passed in 2016 that expand the powers of law enforcement agencies for anti-terrorism purposes. The law requires services such as Zello to register with Russian telecoms watchdog Rozcomnadzor—something it has failed to do.

Zello managed to irk Russian authorities last year for refusing to store user and conversation data in Russia and share its encryption keys with the Federal Security Service (FSB)—the successor to the KGB. It was banned in Russia, but the app managed to circumvent the block.

Now, Rozcomnadzor is reportedly about to expand its campaign against Zello. Russia’s Vedomosti reports that the agency has issued letters to most of the country’s ISPs recommending that they block the app.

The letter mentions an experiment that would see ISPs block 36 Internet subnets, which represents 15 million IP addresses, to take down Zello. 26 of the subnets belong to Amazon (Zello uses Amazon Web Services).

“The subnets selected by Roskomnadzor are not all Amazon’s IP addresses, but they account for a significant portion of the addresses from two large regions of the United States where the company’s data centers are located,” writes Vedomosti.

As it's claimed Zello doesn’t completely depend on the listed subnets, hundreds or thousands of other services unrelated to the app could be affected if they’re blocked.

Last month, encrypted messaging service Telegram lost an appeal against handing over its encryption keys to the FSB.