Hacker collective Anonymous has disabled several Russian government websites including the state-controlled Russia Today news service.
Hackers identifying with the Anonymous collective announced they had launched cyber operations that briefly took down RT.com, as well as the websites of the Kremlin, the Russian government and the Russian defence ministry websites.
RT.com confirmed the attack took place, saying it slowed some websites down while taking others offline for "extended periods of time".
RT's coverage of the situation in Ukraine has been overwhelmingly from a pro-Russian perspective, showing fireworks and cheerful celebrations in the newly occupied territories.
In the UK, MPs said the TV channel is Russian President Vladimir Putin's "personal propaganda tool" and should be banned.
DDoS attacks flood websites with traffic
Internet security expert Robert Potter said the distributed denial-of-service (DDoS) attack used involved multiple systems flooding a targeted website so that no other traffic could get through.
"It's like trying to run five people through a door at the same time," he said.
DDoS attacks are considered easy to mount and easy to defend against.
One simple measure is to switch off foreign traffic to a website, which could explain why it currently appears to be easier to access RT.com from inside Russia than outside.
"DDoS is rarely more than inconvenient," Mr Potter said.
But Mr Potter says we're likely to see more Anonymous "cyber activism".
Anonymous is a decentralised collective without hierarchy or leadership and has been known to take on a wide range of issues, with previous attacks targeting the CIA, Islamic State and the Church of Scientology.
An Anonymous video posted on February 15 threatened to take Russia's industrial control systems "hostage" if the Ukraine crisis escalated.
"There's a real risk of country-versus-country cyber warfare leading to an escalation," Mr Potter said.
"Any cyber activity that's conceptually anonymous and deniable is more likely to be successful as it won't escalate things between countries."
Escalation includes the possibility of a Russian cyber attack on the US.
In January, a US government intelligence brief warned that Russia "would consider" a potentially devastating cyber attack against the US if NATO intervened to defend Ukraine.
Security experts have also warned Russian-linked criminal gangs may be encouraged to target Australia with cyber attacks.
Earlier this week, Russian cyber forces mounted DDoS attacks on the websites of several Ukrainian banks and government departments.
The government of Ukraine has asked volunteers from the "hacker underground" to help protect critical infrastructure and spy on Russian troops.
