The problem was caused by a fault with the .ru domain's global security system, which is meant to protect data from hackers, the Russian Digital Ministry said on Telegram.

But the system failed to do its job, and instead blocked users from visiting websites on the .ru domain, including some of the country's most popular and important sites.

Among the websites that were hit by the outage were Yandex.ru, the top local search engine, Ozon.ru and Wildberries.ru, the online shopping giants, and the apps of the biggest banks, Sberbank and VTB, state-run Ria reported, quoting Downradar, a service that tracks internet traffic.

The outage caused frustration and confusion among users, who were unable to access online services, check their emails, or do their work.

Some users joked that the outage was a sign of the government's crackdown on the internet, or a glorious rehearsal for a cyberwar. About the only thing no one joked about was that it was caused by Ukrainian hackers, as that would not be funny.

The Russian Digital Ministry said it was working to fix the problem, and urged users to be patient.

But it did not say how long the outage would last, or what caused the fault in the first place, but it was not Ukrainian hackers.