Ukrainian authorities have extended the country's sanctions against Russia by blacklisting 1,228 individuals and 468 entities. These include popular Russian social media sites with millions of users like Vkontakte, e-mail portals, software companies and television stations.

Kyiv and Moscow are locked in a bitter sanction counter-measures dispute following Moscow's illegal annexation of Crimea and its support for pro-Russian rebels in East Ukraine. Many Russian media have already been banned in Ukraine.

According to a decree signed by Ukrainian President Petro Poroshenko on May 15 and published the next day, economic and other restrictive measures will be applied against the Moscow-headquartered service Mail.ru Group, the St. Petersburg-based social network VKontakte as well as their Ukrainian subsidiaries.

"About 25 million Ukrainians communicate with their friends in Ukraine and around the world using the platforms that are being locked, now they will be deprived of these connections," Mail.ru Group said in a statement.

Kyiv has also imposed three-year sanctions against Russian internet browser Yandex and its Ukrainian subsidiary.

"Unfortunately, the decision of the Ukrainian authorities concerns not only us. These are sanctions against 11 million Ukrainian users who choose our services every month, and against thousands of Ukrainian organisations that use our technologies and services to develop their businesses," Yandex, which has operated in Ukraine since 2005, said in a statement.

The blockage of the Russian sites will take at least two years to implement at a cost of around $1bn, the head of the Ukrainian internet association Alexander Fedienko told the 112 Ukraina TV station.

Also affected are a number of well-known IT companies and their Ukrainian affiliates (Firma 1C, Parus, Softline Group, Galatiki Centre, Intalev Ukraine, Documents Management, System Integrator APO NVision Group, Masterdata and Ascon).

Russian Firma 1C bookkeeping software, which has dominated the Ukraine market for use by small and medium-sized businesses, denies any active contacts with the Russian company and says the software has been localised, according to Interfax.

Additionally, Kyiv has imposed sanctions against Russian television companies TV Centre, RBC, NTV Plus, Zvezda, TNT, Moscow Media, Rossiya Segodnya and REN TV.

The latest sanctions include blocking assets, temporary restrictions on the right to use and distribute property, trading limitations, bans on withdrawing capital from Ukraine, limiting or stopping telecommunications services and using public telecommunications networks.

Earlier, Moscow imposed a food import ban, which is widely considered as the Kremlin's retaliation for Kyiv signing a free trade pact with the European Union. The Ukrainian leadership is now considering cancelling the visa-free travel regime for Russian citizens and introducing visa requirements.