Ukraine is facing widespread criticism for deciding to block access to popular Russian social networks that Kiev sees as a source of Kremlin propaganda and possible cyber attack.
The Odnoklassiniki and VK social networks, the email provider mail.ru and the Yandex search engine were among firms added to a Ukrainian blacklist that now includes more than 400 Russian companies and more than 1,000 individuals.
Kiev insists the sanctions are a justified response to Russia's annexation of Crimea and support for separatists in a bloody conflict in eastern Ukraine, as well as its use of propaganda and alleged cyber aggression against a number of states.
The banned sites are among the most popular in Ukraine, however, exposing president Petro Poroshenko and his government to potential domestic anger as well as criticism and tough questions from abroad.
"Blocking of social networks, search engines, mail services and news websites goes against our common understanding of freedom of expression and freedom of the media," said Thorbjorn Jagland, secretary general of the Council of Europe.
US-based Human Rights Watch said the move “dealt a terrible blow to freedom of expression in Ukraine”.
"It's an inexcusable violation of Ukrainians' right to information of their choice, and the European Union and Ukraine's other international partners should immediately call on Ukraine to reverse it," the group said in a statement.
Ukrainian media quoted a spokesman for the European Union delegation in Kiev as saying the bloc was waiting for “more detailed information” on the ban.
Russian officials denounced the move as censorship and a crude bid to deepen the divide between the now-hostile neighbouring states, while analysts questioned whether it was legal and technically possible to block the sites.
After Mr Poroshenko made the announcement on Tuesday, his aides accused Russia of launching an attack against the website of the Ukrainian presidency.
“Hybrid war demands adequate responses to challenges,” Mr Poroshenko said on his own VK page, while announcing that he would shut it down.
"Massive cyber attacks by the Russian Federation around the whole world, in particular the recent meddling in the election campaign in France, show that the time has come to act more often and more decisively," he added.
On Wednesday, Mr Poroshenko said the ban could be lifted if Russia ended its aggression against his country, and Kiev officials said that people in Ukraine would not be punished for finding ways to indirectly access the banned sites.