The websites of major Russian news outlets including TASS, Izvestia and Kommersant have been hacked on Monday.
The websites could not be opened earlier, with many displaying error pages.
The press service of the Izvestia newspaper confirmed the hack, adding that everything is currently being done to ensure citizens' access to information, Russia's RIA Novosti reported.
The editor-in-chief of Kommersant, Vladimir Zhelonkin, also confirmed the hack.
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The website of Russian state news agency TASS was also hacked, Reuters checks from several devices showed, with the regular site replaced with an anti-war message and calls to stop President Vladimir Putin’s invasion of Ukraine.
“We urge you to stop this madness, do not send your sons and husbands to certain death,” the message read. “Putin is forcing us to lie and is putting us in danger…It’s not our war, let’s stop him!”
Russia says its forces intervened in Ukraine in a “special military operation” to demilitarise the country.