Ukrainian President Petro Poroshenko has imposed sanctions on several leading Russian social networks and search engines, ordering access to the sites to be restricted or blocked entirely in Ukraine.
Russia reacted angrily hours after Poroshenko's decree was published on May 16, calling it "unfriendly" and accusing Kyiv of censorship.
The Internet companies named in the decree included popular social networks VKontakte and Odnoklassniki as well as prominent search engine Yandex and the Mail.ru Group.
It ordered the "limitation or termination" of access to the sites, prohibiting Ukrainian web hosts from linking to them for a period of three years.
"The challenges of hybrid war demand adequate responses," Poroshenko wrote in a May 16 post on his own VKontakte page. "Massive Russian cyberattacks across the world -- particularly the interference in the French election campaign -- show it is time to act differently and more decisively."
He added that he would shut down both his VKontakte and Odnoklassniki accounts.
More Russia Sanctions
The move opened up Ukraine to criticism frequently leveled at Russia, where media-freedom groups say President Vladimir Putin has stifled independent media over more than 17 years in power.
Russian authorities have also banned or blocked websites and specific content, citing legal or technical reasons that Kremlin critics say are often pretexts for suppressing information or views it does not want made public.
Russia blocked the prominent U.S.-based social network LinkedIn in November, saying it was in violation of a law requiring websites to store personal data on Russian servers.
A Russian official said that users would get around the Ukrainian restrictions, and Mail.ru Group posted instructions on how to do so.
But many Internet users mocked that step by Mail.ru, saying the instructions would help Internet users in Russia gain access to foreign-based websites blocked by the Russian authorities.
The decree adds to a list of mainly Russian companies and individuals subject to sanctions in connection with what Kyiv says is Russian aggression against Ukraine -- a reference to Russia's seizure of Crimea in March 2014 and involvement in a war that has killed more than 9,900 people in eastern Ukraine since April of that year.
With relations in tatters, Russian social networks have been used by President Vladimir Putin's government and pro-government groups to promote the Kremlin's position in its standoff with Kyiv and the West over Moscow's interference in Ukraine and other issues.
Later on May 16, Poroshenko's office accused Russia of carrying out an organized cyberattack on his website in response to Kyiv's decision.
"We have been witnessing Russia's response to the presidential decree that mentioned closing access to Russian social media. The website of the president is affected by an organized attack," the deputy head of the presidential administration, Dmytro Shymkiv, said in a statement.
"The situation is under control thanks to our IT specialists and there is no threat to the work of the website," he said.
In Moscow, Putin's spokesman, Dmitry Peskov, told journalists that Poroshenko's decree was "another manifestation of unfriendly, short-sighted policy toward Russia," and said it "violates the rights of people in Ukraine to receive information."
Russian Foreign Ministry spokeswoman Maria Zakharova called it "politically motivated censorship."
Other entities named in the decree are Internet security companies Kaspersky Lab and DrWeb, as well the Russian media companies RBK, National Mediagroup, TNT, and Ren-TV.
'Freedom Of Speech' Vs. 'Enemy Propaganda'
The sweeping measures drew fire from journalists and rights activists, who called it an attack on free speech.
"Nothing can justify such a blanket ban! Blatant violation of freedom of expression," the Eastern Europe and Central Asian desk of media-rights watchdog Reporters Without Borders said on Twitter.
Aric Toler of the open-source investigation unit Bellingcat, which has produced searingly critical investigations of Russia's role in the Ukraine conflict, said VKontakte groups "are often the best source of information for civilians living near the front lines" in eastern Ukraine.
"So, that's now gone. Great job Poroshenko," he wrote on Twitter.
The well-known Ukrainian journalist Maksim Eristavi noted that the ban places Ukraine in the same camp as several governments widely criticized for their record on human rights and media freedoms.
Meanwhile, influential Ukrainian journalist Yevhen Fedchenko, director of the Kyiv Mohyla School of Journalism and co-founder of the website StopFake.org, came to Poroshenko's defense. He said that banning VKontakte and Odnoklasniki "would be the greatest contribution to protection" of Ukraine's information "sovereignty."
Fedchenko called the social networks "instruments of war" that spread propaganda, hate speech, and "war mongering," adding that they should be "discontinued temporarily."
Peskov hinted that Russia could retaliate, saying that Moscow was "monitoring the situation quite carefully, and certainly we are keeping the principle of reciprocity in mind."
Adding a note of mockery, he said that there were "not so many major information resources in Ukraine that are popular in the Russian-language space."
Ukraine has imposed sanctions on a total of 468 organizations and companies as well as 1,228 individuals in Russia, Crimea, and the separatist-held parts of the Donetsk and Luhansk regions in eastern Ukraine.
The decree was signed on May 15 and came into force upon its publication on May 16. Hours later, the websites remained accessible in Kyiv and other Ukrainian cities.
Speaking on condition of anonymity, a Ukrainian government official told RFE/RL that it would be tough to implement the access restrictions because there are ways to circumvent them.
Russian Deputy Communications Minister Aleksei Volin told journalists in Moscow on May 16 that the move "discredits Ukraine's leadership" and that Ukrainian users would "quickly find a way to bypass any block to access the sites."
With reporting by RFE/RL's Ukrainian Service, Current Time TV, TASS, Interfax, Reuters, AFP, dpa, and Dozhd TV